There's no way to know if anyones 100% clean or not, but I've always used Mike Ohearn as a possible example for my kids. If you take an hour and do some research, he's been competing for 20+ years in both BBing, and PLing. He set teenage PLing records, and his lifting has progressed pretty linier for 20yrs. After he became an adult he has only ever competed in tested events, both Pling, and BBing.
The fact that you can research his Pling numbers for 20yrs, plus look at photo shoots for 20yrs, there's never been a huge jump, and his build, all though big, still seems atainable. his arms and shoulders, don't have the ridiculous size of Pro BBer's. There is absolutly no way to know for sure, but I have a good feeling about this guy. The fact that he's maintained for 20+ years, both build, and pondages, also lends to his natural status. most juicers don't maintain for 20yrs. They come and go, Also his stance, isn't against juicers, he says he trains with them and uses them for motivation. Listen to his radio interview at "Supertraining. TV" and his own web sight.
Now having said all that he deads around 700lbs, squats high 600s, and benches around 500 @ a body weight of around 230 lean, and 270 (fat) when he's PLing. He's tall, mabey 6"4' or somthing. Wheather he's used or not, I believe he's on the far end of what could be acomplished naturaly. keep in mind 20+ years of steady hard heavy training. You can't train for 6mths and call bullshit on this. his father was a competative weight lifter, and i think his mom was a medal holding athlete as well. genitics matter as well
Layne Norton as well claims (and I believe) to be natural. he moves some big weights as well, at a smaller body weight, but he also has been training for alot of years. I think a natural trainer could hit almost as much as a juicer. I just think it will take 5 times as long. Getting strong is a lifelong endevor. Goodluck