What I usually do when it comes to sandbagging or proving sandbagging is if I'm in a league and know someone is sandbagging, I'll look up their historical averages on leagues and print them out along with a few weeks worth of scores to see if they're sandbagging.
I know everyone has a bad week every now and then, but if you go from consistently 150s to consistently 200s or something like that of a big jump, I'll send in the info to the USBC and let the USBC handle it.
I've only had to do this once, and don't activly search out people to report, but if I know its occurring in sanctioned league play, I'll report it.
In any case, most well run leagues have ways to prevent this and this typically only happens in house run leagues.