Actually, with sophisticated programming, it would not be possible for any official to know if there were a program imbedded in a computer to slightly alter the votes--and it can be done without any detection unless a parallel paper trail is produced that is independent of any computer. Many states, including Nevada have electronic voting machines that have a paper printout that you review before you finish, but only guarantees that the computer correctly inputted what you desired. It is no guarantee that the numbers processed later on will not be altered. Officials always take the word of the company who provides the machines, that there are no backdoor scripts to change the results. Can the companies be trusted? That is doubtful given that every major voting machine company like Diebold has connections with high government officials.
Here's what I think is going on. With the advent of exit polling, vote count manipulators are limited in how many votes they can alter. I would estimate that they can only get away with a 5-8% manipulation of votes before showing a significant discrepancy with exit polling--which is pretty accurate since several independent groups do the surveying. But that little percentage is enough to change the results in a close race.
More...