You can watch the box that the votes are put into, you can be present at the opening of the box... But you can't monitor the boxes in transit or be sure that a

You can watch the box that the votes are put into, you can be present at the opening of the box... But you can't monitor the boxes in transit or be sure that any given box, or vote, was tallied properly. It's happened numerous times in the Bay Area that boxes of ballots have been "found" days after an election, with votes curiously inconsistent with others from nearby precincts.