Haha, I guess if he's around you, its not going to be limp!
As for how to tell. Well when he's erect, you should still be able to tell that there's some extra skin that is actually pulled back exposing the head. I guess you haven't seen another so you can't really compare.
Maybe ask him to show it to you when its limp. If he has a semi, you may be able to see some of the head covered by the skin.