I think you need to nail 'em on the day of the contest, or else they get away with it.
Scale is definately a BUILDER's event, and somebody who enters a model he didn't build is very, very wrong. But if the enters, get it static-judged, flys, and gets the trophy, it's a done deal. Just be on the lookout next time.
For decades, people have thought that the USSR's winning F4b models were build in gov-mint shops by teams of skilled tradesmen, but short of bringing in the CIA, it was never proven. When the "builders" got too old to fly and they were replaced by younger pilots, there was strong suspesion.