In the vast majority of Bay Area HOV lanes you cannot pay to be there, that only applies to fasttrack lanes, and in those you can read the occupancy setting of others cars on an overhead screen as they drive though. In both cases you can easily tell, especially in very slow moving traffic who is in violation or not.

Once when bored in very slow nearly stopped traffic during rush hour on a stretch of the 80 with no fasttrack, and in a vehicle high enough to see if there were kids in the nearby cars, I counted a large sample (about 50) cars and found that roughly two thirds of the HOV occupants were in violation.