The FasTrak scanners above the lane flash the occupancy setting (1, 2, or 3+) on the driver's transponder. It's easy to observe cheating single-occupant vehicles because the flashed number is 3 (a toll-free rate).

For automated enforcement, there's prior art in red light camera systems that mail tickets/violations to the registered vehicle owner.

In Washington state, for one, I know that there used to be a phone number posted periodically for civilian reports of HOV violators. That's gone now with just a warning of the fine amounts.