One of humorist (and Maine native) Tim Sample's monologs addresses this: basically, if you were born in Maine, you are a Mainer. Doesn't matter if you were born in Maine and then left the state after 1 day never to return, you would be considered a Mainer. On the flip side, if you were NOT born in Maine, but spent the remainder of your life here, you are "from away".
By Tim's reckoning, Harry Percival Jr. (who was actually born in Florida) was not a Mainer. But you would never have been able to tell.