Force Signature was the ability to sense and identify an individual through the Force. The power was first developed by Scott Borek during his sojourn into the Unknown Regions to search for his brother's killer. The power was then taught to Jedi Master's Mason Quarrell, Pup Kickto and Yoder Gree and Jedi Knight Alec Prime. Identification was usually done on the basis of strength of the Force user, use of the Force and the familiarity of the signature based on previous encounters.
Active-use identification was difficult for a moderately trained Jedi to master. It usually took months or years of identification training to master. It involved the ability to identify a being in the Force while they were actively engaging in a physical activity or actively practicing the Force. The lesser the action, the harder it was to identify. It also was harder to use in more densely populated areas such as Coruscant, Nar Shaddaa or Eriadu.
Passive use is the ability to identify the the signature of a being after they have used the Force or extreme physical/emotional activity. This is much easier for a Force user to learn and is the lower-tier version of the two types. The difficulty of using this technique is the fact that the signature dissipates or deteriorates over time, so a signature is easier to identify the next day than a week later. Passive use was the first of the two techniques discovered and mastered.