I find myself asking nicely to see permits, and suggesting that they make sure to hang or display them every time they use a space, so people won't question them. If they say they don't have one, I ask if they have a disability, then explain how to get one. If they are not disabled and get nasty, I try to whip out my cell phone and get a shot of their license plate as I explain that they are violating a law that could cost them $250!
Just two days ago both spaces at the ATM at our bank were occupied by cars with no tags or placards. The one women I approached got huffy in a hurry, and I did not even have time to flip my phone. Uh, sorry, it may not be nice . . . but I flipped her something else as she cussed and drove away.