I doubt that they do know that. Votes are secret in this country. Ballots are separated from the records of who voted. You can know how many votes were fraudulent, but you can't know who they were voted for.
But statistically, you could figure out the probability and be about right. Obviously, since Milwaukee went heavily for Kerry, the ratio of Kerry votes would match the county approximately...and therefore, Wisconsin was MUCH closer than Ohio's..but then, you already knew that!