Above all, when you ask to join, see if the Sensei checks YOUR background or not. If he accepts you at a whim...hmm. I think its more important for the sensei to study the prospective student then the other way around. If not he could be teaching potentially dangerous techniques to a bunch of loonies or maybe he's just in it for the money (yeah rite, rich aikido senseis, that'll be the day).
But if he seriously gives consideration to take or not to take students, then I guess he has more then just materialistic concerns when he runs the dojo.
Oh... and try not to have any excess 'baggage' when you go there.