We had that problem several years ago with a well-renown contest (which doesn't exist any more). Guy came in and entered a bunch of models . . . noticeable variations of techniques and skills were apparent. Two things tripped him up: One, he bought a model with custom-made one-off decals and the guy who made the decals was present & told us who he made the decals for (not the entrant, but the guy who sold the model to the entrant). Two, he bought a collection of built-ups from a seller here in town, then entered a couple in our contest. The seller had entered the models in previous contests here. The entrant was from out of town. Let's just say none of his entries were very successful after that.
We had another instance . . . a local builder of, say Skill Level 2 capabilities, entered a dozen or so models. One was sporting a Skill Level 4 or 5 paint job of a particular style. Even models the entrant built after this one had sandpapery dull paint and serious Olympic-size orange peel. But that one model . . . world-class killer paint job. Another entrant had a couple of models with similar finishes ~ world class, killer, same colors, same style graphics and techniques. He was a talented 1:1 car painter as well. We asked Skill 2 Guy how he accomplished the special effects of his paint job on that one model. But he couldn't tell us the technique employed. We asked the painter . . . and he admitted Skill 2 Guy paid him to paint the body of that one model. Skill 2 Guy also had a streak of dismal failures in the podium-finish department after that. He quit buying paint jobs.
We now keep a booklet with winner lists from past years and photos of the winning models. Any questions, we look it up. We usually catch the cheats. Unfortunately, not always. Cheaters can quickly destroy the credibility of any contest or show, so it's important to try to discourage them.