We absolutely do know. Study after study has found actual voter fraud to be practically non existent. What does happen a tiny but measurable amount of time: John Smith signing on the line of the wrong John Smith.

Actual, intentional rampant fraud has not been documented.

I’m sorry, but how can you demonstrate that there is no fraud when the identity of people voting is not verified? You simply can not. At best you can say the ballots cast are all for registered voters, but not that they actually cast them.