[Roadrunner383]

I think when a mail-ballot is returned, the voter who sent in the ballot has their record flagged in the voter registration database as having voted. The voter’s party identification is in their voter record, so it’s easy to tally Dems, Indies, Republicans that have voted.

[The Old Hoosier]

That is correct. Of course, we don't know who any of these people voted for, but you have to think an increased number of Republican voters means an increased number of votes for Trump.

Also, if you vote early or absentee, many states let you check online to make sure your ballot has been received and deemed valid.

[Magnum44]

Ok, that makes sense