In Wisconsin you drive to a specific testing facility and, when it's your turn, onto a testbed with rollers under the wheels. They spin endlessly without the car going anywhere until the test is over, when the rollers are locked so the car can drive off of them. During the test they attach a hose to the exhaust.

Engine revs in drive with no corresponding accelerometer output or change in GPS position should be a pretty solid indicator of "I'm in an emissions testing facility right now." Doubly so if there's an OBD-II reader attached.

Where I live, they just plug a thing into the OBD-II port and go with whatever the car tells them. (I drive a '96 Honda Civic.) They don't test the exhaust directly.

Uhm, the emissions test you do every year (I guess in some US states is the same as in EU where you have to measure that yearly?) is different than emissions test that's done on each new model of car before allowing them to be sold and then entered into official specifications.

Remember there are restrictions like EURO5 etc. that impose higher taxes, city environmental taxes and even outright ban cars from being sold on the basis of these specifications.

Yeah, plug in OBD-II reader, get result back.

Some emissions places also do the tube-up-tailpipe check, but the car could adjust exhaust output if it has an active OBD-II connection.

Tests have fixed usage patterns. You detect those while total mileage is low, and you assume it's a test. After X thousand miles, you stop detecting. Job done.

eh, not really. In atlanta, it was once a year, around your birthday. Where I am, outside savannah, it's never. So my car would have no idea if it's being tested or not

That's not the test we're talking about here. This is the official test to get a certificate of compliance and sell a vehicle in the US.

I can second this, once a year in the Metro Atlanta area. Where I went to college they didnt do emission. Many classmates switched their registration so they didnt have to pay yearly emissions.

First of all, that's not how it works. Secondly, being in neutral and revving the engine seems like a pretty obvious red flag for "hey, maybe I'm being tested for emissions" in the scenario you mention.