they put a bit of paper at the exit of your exaust and if it turns a certain colour then you have no cat cause the cat filters out certain chemicals
and you can get an engineer's cert without an emmisions test by any engineer because they are two different things, an engineers cert is just saying the car is safe, eg engine conversion is safe, brakes are big enough for the new engine, the front mount is mounted properly, pod in an enclosed box, no atmo bov
even with an engineers cert you can still get an epa notice, the epa is only about the emissions not weather the car is safe or not, you can have a safe car that doesnt comply with emmissions