How did you do it? In all the years I've worked, I can only think of 3 times that I prevented someone from voting who shouldn't.
One was a water referendum and a man came in an announced in broken English that he was not a citizen. (I was a poll watcher in that election). The ladies at the table discussed it among themselves and said, "He's lived here 25 years, he should vote." They were getting ready to register him when I called the election judge in from the back room and she told the women that he wasn't eligible to vote.
Two was a teen ager who wanted to vote in his brother's name. I happened to know the boys. They were Muslim, as was their father, and they were all named Muhammad -- but one was not registered. I sent him home for his ID. (He didn't come back until the next election.) I got scolded by a friend of mine for turning him away because she thought that he'd vote for our candidate -- I wasn't so sure. And even so, he wasn't eligible unless he came back and registered properly. His mother is an attorney, and I know she'd tell him the same thing.
Three -- a mom asked if her daughter who was home from school could vote. I told her she could and told her what ID she needed. A half hour later the girl came in with the proper ID and I noticed that some of her papers said, "Madison". I asked her if she'd already voted in this election, and she said "Yes, in Madison". I told her that she couldn't vote twice in the same election and turned her away.
That's not many "saves" for years of work, but it's worth it. Of course, in Milwaukee and in Madison they are registering more people than live in a building all the time. Because we register new voters at the polls (a stupid idea IMO) we have no way to check if they are really telling the truth. After every election they send out postcards to the new registrants and about half of them are returned as bad addresses.
How did I do it? In the 2000 election I was a Poll Watcher. As soon as the poll opened I noticed the clerk at the front table writing a voter’s name in the back of the Voter Roll book. The Election Judge shrugged and said that the registration was probably lost or the guy registered too late to get into the book. I looked over at the guy’s Voter Registration Card and it was for another precinct! I objected and the Election Judge reluctantly ordered the guy to go vote in his home precinct. That went on all day and I kept notes on how many I stopped. What they were doing is taking vanloads of people from one precinct to the next voting repeatedly and paying them each time they got back on the van. Our new Republican Registrar of Voters put a stop to that nonsense and now our E-Slate electronic voting system picks it up immediately if someone has already voted even if it was in another county. The important thing to look for as a Poll Watcher is inspecting credentials. You will find voters from different precincts, 50 year olds with cards that say they are 25, birthdates of 00-00-00, and other oddities. Be ready to call the state or federal authorities if they refuse to run a clean election.