You can use blockchain to identify the source of the ballot. If they did it correctly, they could even have a particular dot key for each county and state so they would be able to tell which ballots came from other counties. Essentially each person would have a unique identifier in their voting district.
https://www.zerohedge.com/political/why-trump-will-triumph-pa-litigation this article explains the process in detail. they are going to the SCOTUS and can potentially void all mail in ballots