I can only speculate, but I suspect it is often as easy as going back and matching voting forms with the rolls. (In Iowa, at least, when we show up to vote we have to complete and sign a small form listing our name and address. If memory serves, it's the usual "swear under threat of perjury it's really me" sort of thing. I don't know how common such practices are.) In any case, assuming a similar system, it would often be easy to show even though the poll worker checked off John Adam Smith, it was really John Paul Smith who voted.