WordNet sense 1 (quick to notice; showing quick and keen perception):

observing

WordNet sense 2 (having the ability to perceive or understand; keen in discernment):

perceptive

WordNet sense 3 ((of individuals) adhering strictly to laws and rules and customs):

law-abiding

WordNet sense 4 (conformable to or allowed by law):

lawful

WordNet sense 5 ((often followed by `to') giving care or attention):

attentive