You just said.. that thousands of people had the same hallucination.. with a straight face.
The way you prove that it works is, well, you project into a store or something nearby that you've never been into before.. then you go there the next day and see if it's the place.
Bell's theorem states that atoms, once touching another atom, act as if they are still touching that item, forever. This would tie into the information gathering aspect of astral projection.