As evidence that a whole bunch of people are registered to vote in more than one state, take, for instance, Gregg Phillips, who is Trump's voter fraud "expert" — the guy who estimated the number of fraudulent votes at three million. He's registered to vote in three states.
Nobody (including me) is suggesting they voted twice. All we're suggesting is that being registered in more than one state is not evidence of voter fraud. In fact, it's a fairly common thing to be registered in more than one state. After all, you don't typically call your old state and ask to be taken off the rolls.
How can you tell if you're registered to vote in more than one state? It's actually kind of time consuming, and you have to remember the last address (or at least the ZIP code) you were registered at in each state you've left.
While there are sites like Vote.org that can do a search of multiple rolls, the database isn't current and only updates when they get people to update it.
What you'll really want to do is visit the Secretary of State's website and find the part of it that deals with finding your registration. Asking Google if you're registered to vote in your state is a good way to get there. For example, search am i registered to vote in georgia?
I went to the sites for Georgia, New York and Massachusetts to determine that I am, in fact, only registered to vote in Georgia, where I currently live.