Unmarked car pulls you over and someone in plains clothes steps out of the vehicle, I'm calling 911 and getting out of there. Now if a Highway Patrol officer in full garb steps out, while on an interstate, pretty fair bet, he is what he is. I would suggest in NC anyway, pull over if an unmarked car goes lights on, see if the person stepping out looks like an officer, if not, dial 911 see what they say.
We can argue the merits of "speed traps" or photo camera's at intersections or in the case of my sister's home state of Maryland, speeding traps using camera's only or unmarked police vehicles making traffic stops, but if a blue light is behind you, you best consider pulling over. I know our NCHP use unmarked cruisers on our interstates on a full time, regular basis... Honestly, they're pretty easy to spot. The license tags are all SHP-####. Plus they all have a small antenna plus a GPS round thingy up on the top roof (due to our mountains I believe) of each squad car.