And it can vary within the states. Texas only requires emissions testing in the most populous counties (no point in testing cars in Brewster county with less than 1 person per square mile population density). In the counties with testing, new cars get a 2-year emissions exemption, after which they get tested yearly.

Testing is generally done via the OBD-II port, so the vehicle self-reports it's condition. Vehicles without OBD-II and "other" vehicles use a tailpipe probe to report on exhaust gases.