In my neck of the woods, there are a few law enforcement agencies that run unmarked cars. (I refer to them as the secret police...worked for Hitler) The cops recommend that if you doubt the authenticity of the vehicle trying to initiate a stop, call 911. The law enforcement dispatcher can then confirm the unmarked cars' validity. That, of course, assumes you have a cell phone with you.
"Sometimes, business decisions must be based not on economics,
but on what is right." ~ Ivan Howes