Proof of citizenship and a scannable thumb print would work. Run all the thumb prints through a set of networked machines to look for duplicates and set those aside for additional scrutiny (and potential prosecution), then count the rest.
Sounds great...but more likely google will have a hand in putting the fix in. They are very close with obummer and are trying to get online voting before the election.
Trump should take this election to the courts if he doesn’t win outright, but we’ll miss Scalia.