I've never been checked, but all my guns are unquestionably compliant for the time being anyway. My biggest concern with having a non-compliant gun would be the possibility that all of them would be scrutinized after any sort of involvment in a defensive shooting. Unless they run the serial number, how would they know anyway? Can they even do that without confiscating the gun?
I know a guy with a very non-compliant mini-14. LEO's have handled it on several occasions, and never said a word to him. I know another guy that carries his threaded barreled, folding stocked, full auto carrier-containing postban SA-M7 to all the gun shows, with cops everywhere. Never a problem.