That's a great point, I didn't think of that. Dispatch would probably know if an unmarked car had called in they were pulling someone over. Although not necessarily, at least you have someone on the phone with your location if things go bad.
Indeed. If you have doubts, call 911 and verify through dispatch that they in fact do have a unit on a traffic stop at your location. Describe your car, and yourself to the dispatcher. If they do not have any units on a traffic stop, then they'll have uniformed patrol enroute to your location immediately.
If you can, continue to a QT gas station, or whatever equivalent is in your state. They are extremely well lit, and are advertised as a "safe place" geared mainly towards women in distress, but I guess would work fine for anyone doubting the validity of the "police car" trying to pull you over.
In this case though, before you actually do stop, I'd call 911, and explain to the dispatcher what's going on, why you're not stopping immediately, and where you intend to stop. Of course if the dispatcher tells you, that it is in fact a real cop behind you, then I'd go ahead and pull over, lest you end up with a Glock 22 screwed into your ear socket once you do stop.