I remember reading already at the time of the 2002 election that if electronic voting machines were used to defraud an election, the way you would be able to tell is that there would be a mismatch between the exit polls and the reported vote.
"Two states are critical now: Florida and Ohio. Both states make significant use of electronic voting machines. Both states have Bush ahead, even though exit polls indicated a definite Kerry victory. Because electronic voting machines do not leave a paper trail, we simply have to take the word of the people and corporations that program the machines that they accurately register votes. But why should we take their word for it? Elections are based on transparency, and there is nothing less transparent than a computer running proprietary software. "
He goes on to say that the one thing Kerry must not do, and that is concede before this has been investigated.