Yeah you can be dobbed in by someone, or an EPA Officer / Police may see you, note your rego and pass it onto the EPA. Used to happen to my Dad's 1974 Mini over 10 years ago (smoky exhaust), got several fines until eventually they pulled the car in for an inspection and said if it wasn't fixed they'd deregister the car.
Here is the procedure the EPA uses to test vehicle noice.
Based on what you said, the EPA has determined the GTR to make "engine speed at maximum power" (ESMP) at 6800rpm. The Department of Environment and Conservation (DEC) holds the ESMP values for cars and bikes.
//Edit: Yeah I just downloaded a copy of that DEC manual and it states for an R32 GTR ('91 onwards), max power 6800rpm, test speed 5100rpm. Doesn't state the imported cars, but it states for cars not listed, the default test speed for a 6 cylinder would be 3200rpm. So if you have a pre-'91 GTR, I'd be pointing that out and asking them to test at the lower speed.
Vijay