The empirical solution would be to administer the mushroom to people who are not equipped to correctly predict how an actual tiny person would physically interact with certain situations (idk maybe something involving surface tension and liquids of different viscuousities) and instruct them to report their observations. This would then either demonstrate that the tiny people do not obey the laws of physics or that they follow the subject's imagined laws of physics and are therefore hallucinations.

Also of course the tiny people do not seem to causally interact with their physical environment in any measurable way (at least to observers of the test subject) so that already strongly points towards hallucinations but that kind of thinking isn't how you get a grant.