[@KatherinWinter] From the soul eater wikia,
Those with Soul Perception have the ability to perceive the souls of other individuals and, depending on their skill level, can "read" a person's soul to various degrees. At a basic level, one can simply sense another soul/presence, though one of the various difficulties in using this ability is getting through the "noise", requiring a great deal of focus to achieve. At the pinnacle of a user's Soul Perception, one could assess an individual's Soul Wavelength, personality, and even capability and type of soul as well as emotional states.

http://souleater.wikia.com/wiki/Soul_Perception