I think this is a big source of fraud, and there's no way to combat it without a monitor in every single precinct.
The tip-off is precincts that have 90%+ (or even 100%+) participation. It's practically impossible.
As long as the poll worker has access to a list of registered voters, they can manipulate the system. We have the technology to block access except to a registered voter, but don't have the will to impose it.
Last elections there were precincts that had >110% turnout, with EVERY SINGLE VOTE FOR OBAMA.
Not peep one from the RNC, and only a squib in the MSM.