Can you help me understand how we'd know if a ballot were cast after election day, if received after election day? I understand postmark is one such form of "signature of authenticity" (in a sense).
There are (generally) three ways to vote. In person, absentee via some form of ballot drop off, and absentee by mail.
In person and absentee ballots dropped off can be trivially found to be legitimate (timewise). You have them by 8PM or you don't.
For mail-in ballots, states have different rules. Some states allow ballots received by the 12th or later, as long as the postmark is on or before election day. Some states require ballots to be received by election day. PA is under a microscope because ballots postmarked by the 3rd, but received by the 6th may or may not be counted, pending court decisions.
Those ballots, however, are not included in current vote totals as reported by the state, so Biden won PA without those votes (and beyond the margin required for a recount). They'd just further extend his lead.
Are there others at play?