Emissions tests in the US are at the state level, not federal. For instance, the state of Virginia has emissions tests. However, the state of South Carolina does not require a test.
Any surprise that emissions tests are required in Fairfax and Arlington, but not in Bland County? Largest city in Bland County has a population of 75; it is a different world from the DC suburbs.
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.