Won't be hard at all. First the OH version of Ronnie Earl/Eliot Spitzer convenes a special grand jury. Then he subpoenas a sufficient number of voters who switched registration on election day and starts dragging them before the grand jury. Eventually someone will spill the beans, especially if granted immunity.
Nobody wants to be the next Nifong, even if it helps ‘Rats. They don’t want that kind of scrutiny.