I'll tell you why I believe he is currently natural.
Face. You can tell a lot about these guys by their faces and Kevin still has the same weak looking face he did when he was at 180 pounds. When he is juiced up, just like arnold and the rest of them, they all seem to gain mutant faces. It's actually quite strange, looking at his latest vid and seeing such a "childlike" version of Kevin in his face with such a large body under. (The latest vid is the chest training vid, check it out, he looks like a child with a big body).
Weights. He is either lying about his lifts and feigning difficulty or he is not using gear. He would be stronger if he was gearing. 12 reps on 4 plates 11/12 days out of the olympia on the battle video. He is a very strong presser and it would come back a lot quicker. 4 sets of 225 doesn't really sound like the Kevin we used to know (or the Kevin I would expect after 6 weeks juicing).
Number of videos. In my opinion, and straight off the bat I thought this: when Kevin DOES start juicing, expect to see those vids dry up.
Finally, what he said when they asked what he would do after the 8 weeks. If he was juicing, he'd be competing again. "I don't know" says to me "Can I afford / Do I want to juice" because that is really, the next level. He's nothing like pro size and he knows it. He knows to continue, to really push he has to juice and that means disappointing a whole lot of people who actually think his transformation is natural. It isn't. He's almost certainly not juicing right now but what he has done is as has been pointed out here, taken advantage of his previous gear usage. Plus, every video I see him where he is at home suggests he has no money.
Your right, see thats the thing, you will know when hes fully on. Nattty, hyper responder, or muscle memory.. It will be a VERY noticable difference in my book..... Im just waiting for it..
Like I say above, when it happens, goodbye regular videos.