Life Sense or Life Detection was a Sense-based Force power which allowed Force-sensitives to sense the life of others. It was capable of detecting live sapient beings that might otherwise remain hidden from a normal person's senses.
It was in fact easier to sense beings of the dark side, though it was unlikely that the Force-user would be able to identify who the person was unless they were very well known to them. Usually, the detector simply knew that there was a disturbance in the Force and that it was caused by an evil individual. Though at distances it was impossible for Force users to identify how far away the sensed person was, or indeed what direction they were even in, once the detector got within 100 meters of the person he sensed he could sense roughly how far away they were.
Life Sense could also be used to detect whether a Force-user was in a Hibernation Trance.