WordNet sense 1 (unobtrusively perceptive and sympathetic):

discreet

WordNet sense 2 (having or showing a sense of what is fitting and considerate in dealing with others):

tactful

WordNet sense 3 (quick to understand):

apprehensive

WordNet sense 4 (having the ability to perceive or understand; keen in discernment):

perceptive

WordNet sense 5 (showing or indicating careful judgment and discernment especially in matters of taste):

discriminating

WordNet sense 6 (mentally acute or penetratingly discerning):

clear-sighted, perspicacious

WordNet sense 7 (characterized by ease and quickness in perceiving):

percipient