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 ca

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.