To someone who has done mushrooms before, sure, they can tell. But for people with no psychedelic experience, niacin (a common stand-in) causes flushing and some other effects which a patient can tell "is doing something."
IIRC, the famous "Good Friday" psilocybin experiment had some people from the control group believe they were not in the control group.
IIRC, the famous "Good Friday" psilocybin experiment had some people from the control group believe they were not in the control group.