Not all big and strong guys are juicers... Some are just genetically blessed, but a lot of boys in their teens and young men in their early 20s can pack on muscle at a very fast rate because of puberty. And after that, humans can get more muscle mass for decades, but at a slower rate. When I was 16, I was frankly shocked how much muscle I added during off-season twice a week weight training for football. I was already built for my age from farm work and a lot of athletics, but once I started the weights, I got huge. Likewise, in college, I lifted about every 3rd day and was getting ripped. So much so I quit because I didn't want to look like a gym rat... I haven't lifted since, but if I were a pro athlete with a heavy off-season training program, I suspect I'd be a rumored steroid user.
Anyway, what I'm saying is, its not impossible, especially when a man has the bone strutcture for it, to get really big if they work out regularly. Look at Nolan Ryan and Clemens. They were big at the end of their careers, but that was over decades of intense training... It's the guys who suddenly get huge and ripped that are the likely juicers, or the guys who in their late 20's (not naming names) who suddenly get bigger feet.
Another big red neon sign that somebody is juiced is when they lift a lot during the season. With the usual wear and tear of playing 162 games, a non juiced player would be foolish to lift a lot during the season. They'd play too many games with sore muscles not yet healed from the weight lifting.