Not long ago I judged a California Common that was in an Anchor Steam bottle that tasted remarkably similar to Anchor Steam. The cap was different but that would easily be changed. Wasn't sure what to do. It wasn't a big competition (~80 total beers/meads) but the prizes were significant ($75). I gave the brewer the benefit of the doubt and it took first (I did note on the scoresheet that it tasted just like Anchor) . The same brewer ended up winning/placing in some other categories as well. Haven't seen his name on any of the other competitions.
I still wonder about if I did the right thing, but it was the best beer.
What would you have done?
I would rather let a cheater get through than falsely accuse an honest, skilled homebrewer. That would really piss them off and I imagine the bad vibes would spread to their friends too.
If you brought it to my attention in my competition, I'd probably make a note and tell you to proceed judging. I don't think that's much evidence of cheating given that cloning commercial examples is fairly common, reusing commercial bottles is very common, and California Common is basically defined as Anchor Steam. The guidelines even say this. So if the goal is 'brew beer that tastes like Anchor Steam' you can't use the fact that it 'tastes like Anchor Steam' as evidence of cheating.
The reality is that this sort of cheating would be very difficult to catch and prove. If it were a small club competition and we didn't know the brewer, maybe give them their award and try to reach out. If they're cheating, they'll probably not like the proximity because you'll get to know what kind of beer they really make.
I also think entering commercial beers is less likely to be successful than people think. Commercial beer isn't always to style and may be influenced by factors such as supply contracts, process limitations, etc. It also may not be at peak freshness.