One of the things citizens can do to discourage "unmarked" LE units is to not pull over for them. This. of course, depends on the state. In my state of La., there are requirements for police vehicles...
Flight from an officer; aggravated flight from an officer
A. No driver of a motor vehicle or operator of a watercraft shall intentionally refuse to bring a vehicle or watercraft to a stop knowing that he has been given a visual and audible signal to stop by a police officer when the officer has reasonable grounds to believe that the driver has committed an offense. The signal shall be given by an emergency light and a siren on a vehicle marked as a police vehicle or marked police watercraft.
If a vehicle in LA. is not marked "POLICE" then there is no requirement to stop until such time as a marked unit appears.
I don't pull over for unmarked vehicles.