I've read in a few different interviews that Arnold called him up and asked him if he used any juice. He said no. I see your point about him using, I just wanted to point out how the general public views a physique like that. Most average people would say it's not possible to get to that without juicing, and he's not even that big! On the street, to a normal person, maybe.....in the gym, not even close.
And as far as comparing him to his physique in fight club, he himself stated that he dropped his regimen the minute filming stopped on AM hist X, and that he basically "delfated."
Besides, Norton is a pro-environment left winger, do you think he'd be caught dead doing a cycle?