This is such a complicated and loaded question it's not really easy to answer. If Jennifer Garner says that Ben gained 30lbs of muscle to shoot and movie and is at 8% body fat, we can make the assumption that he used steroids. No one, except a skinny kid in high school who has never worked out, will gain that much lean mass in a short time. Not all people on steroids are huge. Steroids dramatically help with recovery and put you back in the game with rest periods.
It is well known that a lot, if not all, top level performers are dopers. Who are these people? Cyclists, runners, sprinters, swimmers, power lifters, etc. Are they all huge? No not really. Steroids allow a person to train a lot without requiring down time. The homeless guy in the video could very well be taking steroids, but we can't easily tell.
The most obvious steroid takers are the ones that are HUGE. Look at the contestants in Mr. Olympia and you will immediately see that these people, without question, are all on steroids. The line becomes murky with smaller sized athletes. The way you can usually tell if someone might be on steroids and they aren't large monster sized people, is by how they train.
Natural people (natty) usually need up to 48 hours per muscle group between heavy lifting. Like if you do a heavy bench press exercise on Monday, you don't want to repeat that again at least until Wednesday. If you squat on tuesday, you don't want to squat again until at least thursday and so on. People on steroids have routines that usually look a lot more insane. Bench press M,T,W,Th,F, etc... and their sets (the amount of work they do) is a lot higher than a natty person.
There is also the issue with age and height and natural muscle limits to fat storage. A person who is 6 feet tall, will have a very specific limit of how much lean muscle they can put on naturally. This is the same for people who are 5 feet tall or 5 feet and 6 inches tall. Virtually every height has a natural upper limit of how much muscle mass you can carry. No matter how hard you train, you will never pass that natural limit without steroids. That's just the fact of life. There is a good write up about this from lyle mcdonald (look him up, he is a sport nutritionist).
Not only is height the limiting factor, but age is as well. Once you hit your thirties, the good stuff in your body that allows you to recover quickly and gain muscle starts to drop like a rock. Well maybe not that quickly, but it's still a nice gently downward slope. It takes longer to recover, longer to gain muscle, and leaves you with less energy to workout.
There is a pretty good picture GIF from /r/wheredidthesodago about a guy in his 70's with a some decent muscle mass, found it: http://i.imgur.com/wRHRqCv.gif that is probably a clear example of steroid use.
KEEPING LEAN: It's really hard to stay lean and vascular while maintaining low BF. For most natural competitors, they spend a lot of time dieting down to low body fat percentages. The natural competitors spend months to become stage ready and when they finally step on the stage, they are in the best shape of their lives for a very short time. They are also usually very dehydrated and have very low energy levels. This is because your body is naturally trying to survive and it thinks that you are dying / starving to death. Your body usually lowers its metabolism to try and save you! It wants to keep the fat on your body, this is what millions of years of evolution has led to. If you see a really built guy who is also really lean, you can surely bet that he is on steroids. Keeping lean on steroids is a lot easier then doing it while natural.
That being said, if you have the right hook up steroids can be quite cheap. I wouldn't be surprised if that guy has a supply chain with good shit for cheap.
