Most of the time I get stuck in a do-loop and the person is gone before I have a chance to make a decision. That's probably a good thing, looking at it from a cost/benefit analysis there is no way anyone should ask such a thing. But I also don't want to pretend this obvious thing doesn't exist because it's an elephant in the room! What if the said person expects me to comment on it? What if the person thinks that other people avoid them due to this non-typical characteristic? I certainly don't want to contribute to that either! I have no idea how to approach these things...
My main goal is to treat them like anyone else and I saw someone with a broken leg I'd ask. I forget the comedian but they cover the topic of handicapped people feeling left out because they don't get commented on, they want to feel equal. I think it might have been Carlos Mencia (yeah, yeah, I know, dude steals jokes) but the point stands - if we want to treat people equal aren't we obligated to ask?