Short answer: they don't. Look at how it is done by Oregon. They ship ballots to the homes of anyone who has registered to vote. This means the person who receives the ballots can fill them out. Ballots can end up at the wrong house. They supposedly "compare" signatures on your voter registration to the one on the ballot. Lets be honest, though, when they are opening hundreds of thousands of ballots on election night no one is comparing signatures to voter registrations. Vote by mail is how Democrats in Oregon ensure election outcomes. They get the force of Broward County tactics without any accountability, under the cover of a system with no integrity. Vote by mail destroyed a once reliably conservative state in Oregon. Don't let them do this to your state as well.