I think the phrasing may seem somewhat imprecise, but I believe the way it is applied in practice is fairly simple: Can one look at the person and say "that is a gun" or "that is a knife?"
An unpublished WI Appeals Ct. case that I found, and which relied on Mularkey, involved a man who had a 5-inch bladed knife with a wooden handle in his boot. The wooden handle was visible outside of the boot and the defendant argued that, consequently, the knife was not concealed. However the court concluded that knives are not the only objects with wooden handles, so that while the object was in a sense visible, it was not readily identifiable as a knife. So I would venture to say a key test is that if an average person can look at you, regardless of how you carry a weapon, and say "that is a gun, knife, or other weapon" then the weapon is not concealed.