Master, you'd be surprised! I had a MARKED CAR do that to me, and the guy just got behind me 20 seconds before! I actually saw him coming and was on the process of going from the left lane to the right side when he pulled to my left, on the shoulder, siren on, gesturing like crasy and all! They're not all 100 watts!
If you're afraid, in the middle of nowhere, and the guy isn't in uniform, not stopping might not be too bad, as long as you slow down, put your 4 ways so he see you acknowledge, and don't do anything hinting you want to evade him, then a judge should be understanding. But DO call 911/emergency! They should be able to confirm within minutes if the guy after you is genuine.
If you're a little but more brave, you stop and ask him for his badge and ID, tru a small window opening.
Nowadays, you can't tell the difference between an unmarked and a normal car. Unmarked are no longer the "el-cheapo" with black weels, no hubcaps, no chrome... Here in Gatineau, they have cool Dodge Charger with tinted windows! Unless the guy start the strobes, you don't even see them. Some even have Japanese cars!