Hmmmmm. Interesting question. I don't know if you actually could blame someone for taking drugs just from visible side effects. Some possible visible side effects from
taking steroids are acne, "roid rage," mood swings, personality changes, gyno, baldness, yellowing of whites of eyes, etc. but those can be explained away and can also legitimately happen without steroid use as well. Most of the possible side effects of steroids are internal. If someone is getting unusually huge quickly it can cause suspicion, but really it's not very solid evidence.
As far as just regular supplements like
protein powder or creatine, I don't know that there is any way to really tell that someone is taking them.