These are Mail-in Ballots. All supposedly filled in by individual voters using their own pens and mailed in. Analyzing the ink using a Mass Spectrometer will identify the unique chemical composition of the ink (like fingerprint). If it matches the ink used by the voting machines (yes, these machines mark ballots), they are phony. Also, 10s of thousands of voters could not have all used the same pen.

If mass produced and copy machine or printer was used all you need is a loupe or magnify glass. There will be a pixel pattern . It's one way we check for counterfeit artwork, money and such. Even if the original was hand filled the pixels will still show up. Now if they used a professional printer service. It could be a different story.. I know that Biden and Harris had done business with printing companies in most of the states because I've seen the receipts. Follow the money. FEC is very revealing.

A great way to analyse if the same pen / ink type filled out a subsequent stack of thousands of ballots. Which is already 99.9% likely but it doesn't hurt to get more indisputable forensics backing up the case that's being made.