"If someone says they have had what they call a religious experience on LSD, how can we know that such is not the case and ascribe it solely to the drug?"

Just to clarify, I mean that when a person claims they've had an experience they believe was of a supernatural nature while under the effects of LSD, it is impossible to write this off with 100% confidence as a mere mechanical effect of the drug on the brain and deny any possibility that what was expererienced might possibly be of a supernatural nature as well.
(though personally I would instinctively be very, very, very cautious about accepting such a claim under such circumstances, I couldn't flatly say it was not so with 100% confidence).