[Yo-Yo]

However, the ballots have already been separated from the envelopes, so there is no way now to "uncount" the illegal ballots.

[LilFarmer]

Signature matching.

[freedomlver]

The ballots in question would all be mail in ballots and there should be a mailing list for those ballots sent. Thats a starting point. From there everyone on the list would need to be contacted and a reason stated as to why they did mail in voting. If the reason they state is cover then the ballot could be disqualified.
Now the question is, can the voters be disqualified by phone? How can we know the person on the other end is the person that voted?

I think a simpler way would be to run an ad campaign and say that anyone who voted mail in will need to come to the nearest polling station to verify. Once they show up, you know they weren’t confined to their homes for any legit reason unless they healed suddenly.

[ClayinVA]

There may be a way to shave off ballots. Throw out absentee ballots of confined voters per precinct based off of the ballot ratio. That would hit places like Madison where Biden won 70/30 hard and you can find enough votes to change the election results. It would limit the fraud to mostly dem areas.