We use the OBDII emission check system in TN, at least in Nashville.
They use a mirror to check for the presence of a cat, if not found, it's an instant fail. Otherwise they hook up the computer to the OBDII port and do the readiness checks. If you fail any of the checks, you fail.
I think in TN vehicles up to 2000 can have two readiness checks in "Not ready" and still pass, past 2000 only one check can be "Not ready". Not ready is what is set when you clear all trouble codes on a car.
What is nice is that if you do fail, your registration is automatically extended by 30 days to give you time to fix the problem. You just have to keep your current registration and the paperwork from the testing facility together in case you are stopped.