It's almost impossible to prove that something was rigged in the sense that you find enough fraudulent votes to turn the election. Even if you did, the election is already certified. But even one person's vote being lost is evidence of, at best, negligence, and they found a bunch. Evidence that one person voted that is no longer living in the county or state, is evidence of foul play. Either by election officials, a different voter, or the voter that moved themselves.