In my hometown in Kansas, they use paper ballots that are of the “scantron” variety (kinda like those tests you took in school where you filled in the oval by your selection with a pencil, and a computer could scan the answer sheets and tally each answer). Those ballots were dropped in a locked drop box and then at the close of the election were transported back to the county election office to be scanned and tallied. It would be rather straightforward to have a tallying machine located at each voting site scanning the ballots before dropping them into the drop box.
Real simple, every voter that casts a ballot in person has to sign the “book” showing that they are there to vote. At the end of the day, a simple count of number of names that signed the book vs. the number of ballots cast (and since the ballot box should be locked with no precinct worker having the key, there shouldn’t be missing ballots). If the number does not match, there is a problem.
If a voter decides to walk out without casting their ballot, there should be a specifc place for those tainted ballots to be discarded (possibly a locked box similar to the ballot box, but has an ink roller that marks VOID across the ballot as its inserted to clearly indicate that it should not be counted. At the end of the day the discarded ballots might provide the correction if the ballots cast is less than the names in the “book”.
If the ballots cast exceed the names in the book, then there’s clearly questionable activities in play, and should face heightened scrutiny.