Obviously off-topic, but see the recent case of the Chinese student who illegally voted and then turned himself in.

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.

None of those are voting. If someone cast a ballot who was not allowed to, then how do you know it's happened if they weren't caught?

What is this assertion being based on other then vibes?

Well, after the fact it's realized the person whose name is on the ballot had been dead for months, and you didn't catch who cast the vote because you don't know who they are.

Or a million other scenarios. Really all it takes is stopping to think about it for more than a minute.

I mean if they registered with some identifiable piece of info (SSN for example) then you would see two votes with the same SSN if they were pretending to be someone else and if they dont have an SSN then they arent a citizen.

Murphy's law, arguably. If you can imagine a way it could happen, it most probably did.