What I don't quite get is if a voter can verify that their vote was counted as they intended, how can they prove that it was not if it was not? Or in other wor

What I don't quite get is if a voter can verify that their vote was counted as they intended, how can they prove that it was not if it was not? Or in other words if many people report that their vote was not counted correctly, can we trust them? As a concrete example assume that Trump does not win the next election. But then a large number of his supporters claim that looking at the results online they can see their vote was not counted correctly. Can we prove that they are lying, if they are?