Some states require a witness on absentee ballots, a few require a notarized signature. The voters may or may not notice.
Some absentee ballots have a green privacy sheet you're supposed to fold over the ballot before mailing it in. But that seems superfluous and many can see the point of doing it (which is far from obvious) so they don't bother. And their vote gets discarded.
In any close race (not just the Presidency) we're going to require a lot of trust in the skilled hands and sharp eyesight of industrious volunteers who examine the ballots and toss some out.
Hand-checking the ballots is heroic work, but not inherently trustworthy or accurate even when registered voter request them (much less in states where unrequested ballots are mass mailed to people who used to live at some address on record, before they moved or died).