A lot of seemingly busy celebrities have rather well developed muscles, at least better than mine. So my question is: Is it possible to tell the difference simply by looking at pictures between people who eat healthy and exercise a lot vs people who take the shortcut of anabolic steroids?
Human growth hormone, while technically a peptide hormone and not a steroid hormone, might otherwise fit your criteria at least in spirit. It's been involved in many high profile doping scandals; for example, Ben Johnson, Tim Montgomery, and of course Sylvester Stallone.
In long-term use, it may cause users to develop acromegalic features, the most superficially obvious of which would be a prominent "lantern jaw" and forehead "bossing". The growth of the jaw would also cause spreading of the teeth.
Although I can't find a credible source where Arnold admits to anything other than anabolic steroid use, I have been told by a professor that Arnold Schwarzenegger's face is the classic manifestation of this side-effect.
So for this specific example, yes, there may be some visible signs of using an exogenous substance versus "dieting and exercise".
Apologies for the shortage of citations, I'm apparently limited to 2 links as a new user. This article verifies the use, the biology, and the allegations against the athletes listed in the first paragraph.
I don't think that is possible. What would you like to see? A specific artery pattern or something like that?
I think the possible speculations come from people that are jealous and try to rationalize the difference in muscle growth between the celebrity and them self by claiming steroid use. What they fail to take in to account is that those busy celebrities can have months of free time in between movies so they could quite literally work-out full time and they get to fill that time with the best trainers and nutritionists money can buy.
But then, you don't even have to, this is how Hugh Jackman trained for the second Wolverine movie: http://www.examiner.com/celebrity-fitness-and-health-in-national/hugh-jackman-eats-6-000-caloriesa-day-to-gain-weight-for-wolverine 1,5 hours a day with a really high intensity specialized workout program combined with a 6000 calorie diet...