Put simply? Statistics. Care to explain why you think we “wouldn’t know” despite repeatedly getting an accurate result every time ballots are manually recounted (since every state requires keeping the paper ballots), by members of both parties? Is it that they are all complicit in tallying illegal voting in order to elect members of the other party? Seems like a simple recount is all it has ever taken to disprove that notion..every…time…that claim has been made. And no, it isn’t prosecutions, it is the number of instances discovered to have mistakenly (or intentionally) voted as based on analysis of voting records in states that these proof-less challenges have been made. As in single digits and double-digits that are statistically irrelevant to an election. So I’m curious why you still believe that is a realistic problem, outside of elections being federalized in which case it very much would be possible with zero oversight (unlike state elections who have had 250 years to perfect their preferred methods of voting and oversight).