Police Channel For 2 Way Radio?

ok i dont know if this is legal but i just got for christmas a motorolla walkie talkie it has 22 chanells or w.e its called and what not but i want to know how do i know what channel the police in fairfax couty virginia use? so i can listen to their radios but not communicate with them you know just listen idk if this is legal though ... my dream is to become an fbi agent so i guess i think stuff like this is coo