The obvious way to test it is to get your friend to place a novel obstacle in his way. A soft cushion, a crocodile infested pit, whatever you have to hand.
It won't be part of his mental map and he wont see it so he'll fall over it (or into it, if you're using the crocodile pit).
Of course, when he falls, the sleep-walker will most likely awake and be pretty upset. Just tell him it's for science. I'm sure he won't mind.
Mutate and Survive
"The Bible is like a person, and if you torture it long enough, you can get it to say almost anything you'd like it to say." -- Rev. Dr. Francis H. Wade