Maybe we could start up a conversations with a suspected illegal voter, and question them like we've seen WWII GI's do in movies about the Battle of the Bulge, like, "Who won the world series in 1941, the LA Dodgers, or New York Yankees. If they don't say, "In 41 the Dodgers played in Brooklyn, they don't get to vote.  (would only work if we could understand their answer)