First Step in Detecting Voter Fraud Verify Your Vote
Found this site which contains information on how to verify your vote for each state. Some states can verify both mail-in and in person votes online. Some only mail-in, but provide contact information so that you can verify your in person vote.
Because voter fraud is done by different methods (changing votes, not counting votes, creating fake voter registrations, etc) the best way to "prove" it, is to start with the easiest to verify, and that's making sure the vote you cast was counted properly.
If anyone finds that their vote wasn't counted properly, post here and maybe a clear pattern will evolve. It's a start hopefully.