You're asking for more than just evidence though, you're asking for concrete proof which is very different. You can have evidence that changes your belief - people acting suspiciously their behaviour being consistent with cheating, lying, changing their story, having history of doing this in the past - all of this is perfectly valid evidence that can change your own belief about what's happening, and you should be able to speak to speak about it.

Imagine that we're flipping a coin and it keeps coming up heads every time, and I make a statement that I don't think the coin is fair. By your standards I shouldn't be able to speak until we examine the coin directly and no number of further flipping will convince you that the coin is biased (because it's technically possible that a fair coin does this too). Well, sometimes in real life we can't weigh/measure/scan the coin directly, we can only make judgments based on the flips that come out and we should be able to say that they look fishy.