In my experience, people who use psychedelics are pretty good at distinguishing between what is real and what is a hallucination but that may be due to their preexisting knowledge of the drug and its effects. I would be curious to see (though I would never consider doing this to someone) how a person would react if they were given one of these drugs without their knowledge. I would suspect that a person who has experience with the drugs would be able to pick up on what was going on but one who never tried them would not.

There also seem to be certain features that are common with certain types of hallucinations. I can't speak about some causes but the traditional psychedelics (which all have a very similar effect on brain chemistry) produce pretty much the same types of visual effects in the majority of individuals who use them. That is to say, one can expect to see repeating patterns, geometric shapes, "melting" objects, et cetera. These differ from the hallucinations that are reported with the datura (which has a completely different effect on brain chemistry) where one may see "phantom" objects and people that seem to be real.

This is really a fascinating topic. Good Post.