USAC openly sends emails to members about their target and to-date drug testing numbers, and catches of people.
The only RR I've ever done, they were checking photo ID at the sign in tent. In the US, that means government issued ID. Also meaning, if you're that intent on cheating an amateur bike race, you're willing to pretty much commit a US felony in obtaining illegal ID just to race a bike as a low level schmuck.
As for motors? Nobody at that low level in the US really cares THAT much like some rich Frenchman would put a motor on their bike. Catch the reference there?
Point is, it's low enough level in the 3/4/5 for lots of local things that cheating would be pretty pitiful.
Is it legit? Yes it's legit. They have motorbike escort with time-gap board, people blocking intersections, post results/standings online, etc.....
If anything, I'd say the 4/5 field has some "bloat" of folks who don't race often (like me) and don't accumulate points to advance. But, accumulate fitness. So, sometimes a 4/5 race can be as difficult as a Cat 3 race. Just one lap less.
Most 3/4/5's are going to be in the same local hammer group rides and you'll see them monthly and have a beer afterwards together. So you know them.