If the car attempting to pull you over doesn't look like an official vehicle, keep driving, turn your inside light on and pull over only in a well-lit, populated area. Do not get out of your car, and angle the vehicle so you can drive away quickly if need be. Crack the window, ask to see the officer's ID and pay close attention to his clothes---are they worn? Do they match? If not, put up your window, drive away and call 911.