Well, you could look to see if the cops are behind you. If it's a malicious actor posing in cop cars already, I think there's bigger issues, since most will stop if they see they're being followed by a cop with their lights on.
In my country you could choose not to stop and instead drive to the nearest police station, if you have doubts whether cops are legitimate. They are supposed to escort you or something like that.
In the US you can call 112 to verify it. Also you can turn on you hazzard lights slow down and drive to a safe place, most cops are thankful when you avoid stopping on a dangerous part of the highway like narrow shoulders or ramps.