it's really tough to say. I have a buddy who tested positive after a fight for a smaller promotion out here in california. I know the guy well, and he told me "i didn't use roids at all" and he was definitely not lying to me. He believed it was a combination of supplements that fucked with his system.
It's really easy to believe a guy is lying when he says he didn't use, but I believed my buddy - he was telling me the truth, IMO. (had no reason to lie to me, and we're close enough that he knows i would never speak on those issues publicly had he said "yeah, i fucked up and used")
tough one, but weird shit does happen. Sherk stood in front of the AC and proved the supplements he took were contaminated. The AC obviously refused to admit they were wrong, although they mysteriously cut his suspension short.
it's not always black and white with this shit. there is most def a gray area.