Turn on your flashers, don't increase your speed or try to flee, and dial 911. You will know very soon if the guy is legit. Drive to a busy area, the most hardcore impersonator is not going to pull that bs in a busy 7-11.
Know what the police vehicles look like in your region. Know what color lights they use. If cops are blue, know that. If in doubt call 911, they will find out if the guy is legit.