Being marked as a beginner for a given game does not mean they never played that game elsewhere.
Being a beginner for a given game doesn't mean they can't transfer knowledge from many other boardgames already played and mastered.
Being a beginner doesn't mean they can't be better than a veteran by getting very quickly what to do.
In doubt, some have the courtesy to tell a beginner that they can offer a bit of explaining of rules (or even of moves) if needed. The answer would tell you if they already played.
Another solution is not to play beginners but only very strong players in order to be sure to be crushed with total honesty.
Personally, I'm daft enough that I can be defeated by a beginner at almost any game I play