To answer the general question, of "if somebody got away with X, how can we know it happened?":

If somebody robbed a bank at gunpoint and was never caught, can we know it happened? Obviously yes.

What if somebody "merely" embezzled from one, and there's a "hole" found in the banks books and money missing, but nobody who did it or how? Still, I'd wager yes, one coudl tell by the results.

What if somebody used illegal means to get leverage on some stock buying/selling, it became know, but they weren't punished and got to keep their profits? They got away with it, but we do know it happened.