Any assister is required to sign an affidavit that they rendered assistance in accordance with the voter's wishes.
About all we could do at the precinct level is flag a ballot which is missing that affidavit and/or has an obvious signature mismatch with the registration book, refuse to tabulate it in the precinct results, and turn it into the county canvassing board for further examination. Personally, I would not hesitate to do so.
Thanks for answering, and thanks for being a precinct judge. Your firsthand knowledge corroborates what I assumed. The system likely depends on the honesty of assisters, who also get to interpret whether the voter is compos mentis, and actually the registered voter. Since it would be nearly impossible to actually check up on every absentee ballot, I would like to see the laws requiring the assisters to be the voters’ next of kin, guardians, or holders of power of atty.
Maybe the instance of this sort of fraud is a lot lower than I assume. My suspicion is that in some areas, political operatives work through nursing homes with “cooperating” staff to turn out lots of votes. This could be a blockbuster subject for one of the O’Keefe hidden videos.