1. When the person he voted for showed up at the polling place and tried to vote -- at which point the real person would have been told that he had already voted (they keep track of who has voted). At which point the real person, and the poll worker, knows that something fishy is going on.
2. When the imposter's signature did not match the real person's signature (I'm not sure if this is checked in all states, but then again it is not my task to disprove hypotheticals).
But so far the right can't even come up with an example of either of those things happening, so a logical deduction would be that it is not happening.