How would they know? Michigan doesn't register people by party affiliation. You are either "registered to vote" or you're not.
I'm registered to vote, and no one will know how I voted until they take a peek at my ballot by pulling it out of the absurd "privacy" (unsealed) envelope after I mail in my absentee ballot. And if they don't take a peek, then no one will ever know how I voted. My ballot will be fed into the machine, which will tally it up without knowing who I am.