If they have a felony conviction on their records, their names are on a special list which is at each polling place (covering that municipality). Their names also should have been purged from the poll book. In actual fact, the person registering new voters has the felon list and is supposed to check it when each new person fills out registration papers. Nobody else sees the list — just the registrar.
In actual fact, however, if it is a busy election with long lines (like the 2008 Presidential) it would be easy for someone to slip through the cracks. It’s one more thing for the registrar to check, and most of the polling places rotate jobs every hour. Therefore, a new person at that duty station may not be up to speed the first half hour of registering people.
There were a couple of people caught in a previous election and prosecuted this past year. Of course their votes had already been counted.
The law in Wisconsin is that a felon may not vote until they are “off paper”, IOW they are no longer on probation.
I passed this article on the the folks in charge of overseeing elections in Fitchburg. They hadn’t heard of this, but said they’d be meeting to figure out how best to respond.
At least one town will be ready to deal with this stunt.