Speaking only for California, on the ballot stub is a box code with a number below it and “A” or “B” for page 1 and page 2 of the 2-page ballot. A box code reader shows the box code matches the number/letter. I believe this same code is also on the ballot itself (but didn’t think to look). I don’t know if they track what number is assigned to whom.
I do not recall any identifying markings on my Florida ballot.
But at the time I was not looking for it so could have missed it.
I mailed my absentee ballot 2 weeks before Nov 3rd.