A Force-sensitive, also known as a Forceful, Force-users, and Force wielders was a being who was highly attuned to the flow of the Force. Force-sensitives could, with training, learn to sense and manipulate the Force. Force-sensitives who gained powers and abilities through the Force became Force Adepts.
The Jedi Order took Force-sensitive children at a very young age (typically until the age of three, but preferably earlier) to start training them to become Jedi. A notable exception to this was Anakin Skywalker, who was accepted for Jedi training when he was just under 10 years old, which would ultimately prove to be the downfall of the Jedi.
A midi-chlorian count through special testing could determine how strong one was in the Force.
Trained Jedi could sense high levels of the Force in individuals. The presence of Force-sensitives in a vicinity could also be sensed. On rare occasion, such Force-sensitivity could be detected emanating from a powerful "vergence in the Force". If, however, one who was powerfully Force-sensitive―such as Palpatine, Darth Zannah, or Anakin Skywalker―willed it, even subconsciously, they (or perhaps even the Force itself) could hide or "shroud" them from detection in the Force. While Darth Plagueis had wondered how such a powerful Force "presence" as was centered in a youthful Palpatine might possibly have eluded him, so did Palpatine himself later wonder how he'd sensed absolutely "nothing" of young Anakin Skywalker's presence in the Force when the boy had been a guest in his Coruscant apartments.
Jedi Master Ki-Adi-Mundi, also, was left to marvel (the year following a mission to Tatooine in 33 BBY) how he possibly could have failed to detect the presence of young Skywalker―a being who, "absolutely teeming with midi-chlorians ... may very well be the Chosen One"―for Skywalker remained for him "a beacon of power...": "How could I have not sensed the presence of one so strong with the Force?" he continued to wonder―a case of undetected (shrouded) identity that disturbed the venerable Cerean so profoundly as to cause Mundi to question his own "perceptive abilities ... Tatooine cannot be a coincidence. I do not believe in coincidences ... but could [Skywalker] really have been conceived by the midi-chlorians, as Qui-Gon suggested? And if he is the Chosen One, destined to bring balance to the Force, why do I sense some danger about this boy?"
That powerful Force-sensitives could go undetected was certainly known to the Jedi High Council. Obi-Wan Kenobi, as a young Jedi Master, once reflected that the Council, in fact, "had long suspected that the Sith knew how to disguise themselves and pass undetected by Jedi." Though he was not a Sith, this rare ability to go undetected in the Force was also held by the son of Dark Jedi Xanatos, Granta Omega, who moreover possessed the rare power to appear to some Force-sensitives as a Void in the Force, a "Force blank."
To achieve higher levels of accuracy in Force-sensitive detection for Jedi candidacy, trained Jedi performed special tests to measure midi-chlorian counts in cells (such as Obi-Wan Kenobi, at the behest of Qui-Gon Jinn, did for Anakin Skywalker).
There were also other testing technologies developed to hunt Force-sensitives during the Great Jedi Purge.
Species with a high number of Force-sensitives included Humans, Kel Dors, Ithorians, Twi'leks, Zabraks, Nautolans, and even the rare and mysterious Duinuogwuins. Species producing low numbers of Force-sensitives included the Wookiees, Jawas, Hutts and Caamasi. The Neti, Anzat, Korunnai (a Human tribe), Vahla, Red Sith, Miraluka, and many other species were entirely composed of Force-sensitives. Gotals may all have possessed the ability to sense the Force, although they had little practical use for it. Jungle Felucians, as a species, were all Force-sensitive, but none were known to have become Jedi.
A creature or object did not necessarily need to be sentient to be able to possess Force sensitivity. Jedi Master Qui-Gon Jinn once gave Obi-Wan Kenobi a stone from his homeworld's "River of Light" which by unknown means appeared to be Force-sensitive in Kenobi's opinion.
Creatures such as the vornskr and ysalamiri of the planet Myrkr were other examples of non-sentient creatures who were considered Force-sensitive. An ancient mynock species was even rumored to have established a Force-sensitive run civilization relying on dark side energy. The Qom Jha and Qom Qae was slightly Force-sensitive and could communicate through telepathy with any being who was Force-sensitive.
Certain species, such as Khommites and the Humans of Jabiim, openly feared or detested those who could be touched by the Force. This was especially true of the Yuuzhan Vong, who not only could not feel the Force, but were entirely unaffected by it. The Rakata had lost their sensitivity since the days of the Infinite Empire but held experiments to rekindle it in subsequent generations. One offshoot of the Rakata, the primitive, mutated Flesh Raiders, had one known member during the Cold War that was Force-sensitive.
People achieved Force-sensitivity by three known means. They either inherited it from a Force-sensitive progenitor (which was most often the case), acquired the sensitivity through random mutation or evolution, or, in rare cases, one was artificially imbued with sensitivity to the Force, such as was the case of the Reborn and Sith Cultists.
Darth Plagueis, a master of midi-chlorian manipulation, lived during the century leading up to the Battle of Naboo. Obsessed with eternal life, Plagueis experimented with ways to cheat death and create new life from the Force-sensitive-determining midi-chlorians.
Plagueis' experiments in creating life may have, in a sense, succeeded. For, according to the Sith Lords, it was Plagueis―having conceived and initiated his plans well before succumbing to death―who influenced the midi-chlorians to conceive Anakin Skywalker. In reality, however, Plagueis' influence in that miraculous conception had only been an indirect one, as it proved to be the Force itself "striking back" for the Sith Lord's blasphemous meddling in the processes of life and creation that caused the fatherless conception of the Chosen One.
Latent Force-sensitivity could also be artificially activated or magnified; some members of Palpatine's Dark Side Elite benefited from this process.
The Force, being far more powerful than any machine in the galaxy, had the ability to be used as a weapon by the Sith, or as a tool by the Jedi. In either case, the Force gave Force-sensitives the ability to perform actions that would have otherwise been considered impossible. For instance, Jedi Master Yoda had the ability, even at over 800 years old, to use the lightsaber combat form of Ataru, demanding incredible acrobatics and speed. This was in no small part due to his amazing power in the Force. Some Force powers included healing, psychokinesis, increased physical abilities, foresight, and various offensive techniques, such as the notorious Force lightning.
Force-sensitives were considered to have an unusual amount of luck, lucky guesses, and correct decisions; this was how droids could recognize Force-sensitives and enlist them for the New Jedi Order. Most untrained Force-sensitives still had raw Force associated talents, such as improved reaction time, physical strength, an affinity to animals, weather or certain other aspects of the Force.
For example, due to his raw Force abilities, Anakin Skywalker was an accomplished pilot at the age of ten, despite a complete lack of formal training, and was able to successfully compete in pod races, despite it generally being considered impossible for a human to do so, due to the extreme reflexes required. Another example was a young Galen Marek pulled Vader's weapon out of his hand with telekinesis, without any Force training. As a child, Asajj Ventress was also able to use a powerful Force Push to force a Weequay raider away from herself with no training.
It was commonly considered near impossible to wield a lightsaber without a sense of the Force. This was, however, an exaggeration; though the unique properties of a lightsaber made it more difficult to effectively wield than a standard melee weapon, non-sensitives successfully utilized them on numerous occasions. However, Force-users were generally more accomplished combatants, able to use the Force to perform feats with their weapon that a non-sensitive individual would not be able to match.
It did, however, require Force sensitivity and, except in rare cases such as Jaden Korr, some degree of training to successfully build one.
Some Force-sensitives such as Streen were known to have drastic side effects for possessing Force sensitivity, although not having the knowledge to properly control it.
In the Old Republic era, Force-sensitives were typically discovered by the Jedi at a young age and enlisted for training as Jedi. Individuals discovered late enough to have developed family ties and friendships with others were not generally trained; an example of this was Alexi Garyn. This rule originated after the Exar Kun War but was not strictly enforced until after the Ruusan Reformation.
Meetra Surik had a gift of attracting Force-sensitives towards her, such as Atton Rand, Mical, Brianna, and Mira.
During the Mandalorian Civil War between Mandalore the Avenger's faction and the Hidden Chain splinter faction, the latter's leader, Field Marshal Heta Kol, attempted to use a holocron, created by Darth Nul, to amplify the abilities of Force-sensitives deemed too weak in the Force to be trained by the Jedi or Sith. She hoped to draft these individuals into her army to use as Force-sensitive Mandalorian shock troopers. The process was ultimately successful, but many of the Force-sensitives had been captured during Hidden Chain raids, and were reluctant to fight for the Field Marshal. The plan ultimately failed when the holocron was retrieved and the Force-sensitives liberated.
Beyond relaying to the Jedi High Council the prodigious test results of a theretofore unsurpassed midi-chlorian count, Qui-Gon Jinn discovered Anakin Skywalker through the boy's prowess at Podracing. Anakin himself once believed as a boy that he had a "string of lucky guesses," when actually these guesses were early indicators of the Force. His mother Shmi Skywalker noticed these anomalies in her son as well, and described Anakin Skywalker as "special." Indeed he was, for Skywalker had no biological father.
Other potential Jedi had been discovered in roughly the same way - when there seemed to be an unusual amount of luck associated with an individual. When Luke Skywalker was looking for candidates for his Jedi Academy he told the droids R2-D2 and C-3PO to search for people with an unnatural amount of luck. Although Han Solo was not known to be Force-sensitive (and, in fact, was skeptical of its existence before meeting Luke Skywalker), he was known for his self described "Smuggler's Luck". This lead his daughter, Jaina Solo, to speculate that he may potentially have a mild form of Force-sensitivity.
During the time of the Galactic Empire, Force-sensitives were known to be taken in to be trained as Inquisitors, Emperor's Hands, or otherwise. Most Force-sensitives discovered by the Empire however, were killed due to three possible factors: the threat of other powerful Force wielders was a fear of Palpatine's and as such most were simply eliminated on sight, discovered alien Force-sensitives were likely killed upon discovery as Human High Culture would suggest, or some were eliminated due to lack of sufficient power during vigorous "tests" performed by Imperials.
Although untrained, Imperial Colonel Dyer was Force attuned.
In some cases, such as those of Fionah Ti, Vaner Shan and Theron Shan, children of Jedi and other Force-sensitives were incapable of using the Force, a rare and unexplained phenomenon that occurred in random generations.
The terms Force-user and Force-sensitive were distinct in meaning. Force-sensitive denoted anyone with Force power, latent or active, while being called a Force-user implied that the individual had at least some understanding and control of their abilities and was able to use them to accomplish specific tasks. The latter was also used in a more specific sense as a general term for Force-trained individuals who did not belong to either of the two major Force traditions, the Jedi and the Sith. Another term for Force-user was Force Adept, for dark-siders sometimes Dark Side Adept (not to be confused with the Imperial title and organization proper).
The main form of detecting the Force through midi-chlorian count was lost to the Jedi because of the Empire's destruction of documents and the Great Jedi Purge. The head of the New Jedi Order, Luke Skywalker, discovered a different way to reveal Force-sensitivity. In Leia's quarters in the abandoned Emperor Palpatine's Imperial Palace on Coruscant, Luke and his twin sister Leia Organa Solo were doing some tests on the inner defenses of her brain. Luke was using the Force to access certain areas of the brain; as he accessed an area in the very back of the brain, he was rebuffed. They confirmed this to be a natural reflex of the Force-sensitive.
During the rebuilding of Coruscant after it was claimed by the New Republic, Wedge Antilles and the team he was leading discovered a chamber of Jedi-related artifacts possibly concealed by Emperor Palpatine. Items found there included a type of scanning device that could detect an aura of the Force around a person. R2-D2 was equipped with this scanner to aid Lando Calrissian in searching for Force-sensitives to join Luke's new Jedi Praxeum on Yavin 4.
At some point in the future, the New Jedi Order would rediscover midi-chlorians and begin using blood tests to better find Force-sensitives.
Sometimes Jedi went into a trance and found a Force-sensitive by detecting the child through the Force itself. This technique was hard to master and spread the Jedi's Force abilities thin as the Jedi cannot fight or concentrate on anything else combined with concentrating on a large number of beings. The technique also requires time to use effectively, which may not be a good idea when things must be done as quickly as possible.
Besides Sith and Jedi traditions and Imperial dark side cadres, there were other organizations devoted to the Force. These often originated in another earlier tradition, but had left it after a schism or similar event. These could belong to either the light or dark side, or simply exist somewhere in between. Some of the better known examples of independent Force-user groups included the Ysanna, the Witches of Dathomir, the Aing-Tii Monks, the Baran Do, the Fallanassi, the Jensaarai, the Krath, the Sorcerers of Tund as well as many other Force-based organizations. Also, some Force-users did not adhere to any code or creed, but were self-taught or raised and trained by independent masters. There was also a family of Force-wielders known as "The Ones" on the planet Mortis.