How does your opponent verify you have not more than 4 of a particular card in your deck?
In general, your opponent cannot conclusively verify that your deck is legal. As you mention, you can always concede to prevent them from seeing certain cards.

However, this statement that you made in your question is incorrect:


  since one can never prove that I'm cheating, one can't call a judge either


Players can call a judge for any number of reasons, including suspicion that their opponent is cheating. And the fact is that conceding in response to a Thoughtsieze or Surgical Extraction is very suspicious, especially if you are otherwise winning the game.

If a judge wants to investigate a deck construction problem, they have the right to perform a deck check, and look through your whole deck and sideboard to verify that it matches your registered deck. If they do that, you can't avoid getting caught.

In addition, in higher-level tournaments decks are randomly checked, so even if you avoid drawing suspicion you would still have to get lucky to avoid disqualification.



More generally, it's pretty clear that this method of cheating has significant downsides. You have cards in your deck that are completely dead if you draw them, because you can't let your opponent see them. You have to concede if you opponent would see them in some other way, tanking your win rate. And you risk disqualification because if a judge decides to look into it, they can easily prove that you are cheating. You may not even experience a net benefit from this strategy, even without accounting for the fact that cheating is wrong.