According to my biology source, the definitive feature is having a keeled calcar on the inner femoral membrane.
Ok, in English, that means they have a ridge on that skin flap which extends from the bat to the tail and between the feet.
Problem is, there isn't any reasonable way to make an Indiana 'spread 'em' without handling the bat. Which the FWS highly frowns upon doing without a permit. Hence the repeated advice for people to get a bat expert to look at suspected endangered species. Besides, they know what they are looking for. (I'm just repeating what I've heard--I wouldn't know a keeled calcar from a muscle car unless someone showed me.)