They mirror under the vehicle first, to be certain there is a cat if applicable, if not, fail, if there is, they put the smog tube in the tailpipe, but newer vehiocles have on board emission testing devices, they just plug into the port under the dash to recieve their needed information.
I beat it out on my lifted 78 F-150, antique tags, last a lifetime and no testing, don't get it out on the road anyway.