In CA, the way it's enforced is they do a visual check (along with all the OBD and tailpipe checks) every couple of years as part of your registration, and if they see anything with a 'race only' sticker on it, you're screwed. Also, if you have any aftermarket devices installed that have anything to do with the emissions (anything to do with the fuel, exhaust or ignition) they look to see that it has an ARB number displayed. If no ARB number, they'll try and look up the device in their database, and if they can't find it or it shows that it isn't ARB certified, you're screwed.
I've heard of people getting pulled over because an officer saw smoke coming out of the tailpipe, or otherwise had probable cause to suspect that a car is a polluter. They can write you a ticket, but it's generally a fix-it type of ticket that requires you to go down to a referee station and have them visually inspect the vehicle and test it. By then, the parts in question somehow always seem to be gone...
