Vote count machines have electronic memories that can be examined.
My ballot was scanned at my polling place.
Sequences of fraudulent ballots can be identified.
Yes! For example, here in New York State, every ballot is cardstock paper, and is imprinted with a unique serial number that appears on both the ballot and the stub from which it was torn. The stubs are all returned to each county Board of Elections at the end of the night. It's been recorded beforehand which ballots were sent to which districts within each polling place. And of course, the ballot-reading and tabulating machine has a clock. (Does it record when each individual ballot was fed to it? I believe so--it certainly should!)
The net is that dragging in bags of ballots from elsewhere to be tabulated late or in secret--whether they're stolen blanks or specially printed fakes--would be a non-starter in New York.
Our weakness is identifying who it is that has just walked in the door, asking to vote a given name that's legitimately on the rolls. In rural and suburban areas, that sign-in signature had better match. Your local district folks recognize you personally.
But in the Democratic cities (but I repeat myself), where there are virtually no genuine Republicans to question a 25-year-old imposter asking to vote as a 95-year-old nursing-home resident? Who the heck knows?