Force-sensitive

A Force-sensitive, also known as a Forceful, Force-user, or Force wielder, 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 (and other Force adepts) 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.

Text and facts from the “Force-sensitive/Legends” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.