This is what I meant Master Vigil and Flupke, that Astral projection and OOBE is experential. When you experience it yourself, you know.
Sure you could argue that it's a biochemical process(and it actually it is, when OOBE's occur, the bio-electrical activity in the brain is affected and I also believe there are chemical changes) and that the experience was entirely in the person's head. But, to the person who experienced it, they experienced a reality so real, so vivid, so alive, so beyond, that to them it is completely real and an inner realization tells them that it was true.
You could all argue all you want, that their experience wasn't true, but as you cannot experience what they expeience, you couldn't know with certainty whether it was true or not.
But it is not completely subjective, as studies into OOBES's and NDE's have shown. When they experience these, they report floating above and out of their bodies, to be able to see all their surroundings and other people interacting. A common example, in NDE's, is being able to see the operating theatre and everything that is taking place.
In documented cases of NDE, after the person is resuscitated, the person can explain in great detail what they saw was going in the operating theatre or nearby, which would be otherwise impossible to know from their vantage.
In scientitic studies, but unfortunately scientific dogma relegate these studies into fringe science/psuedoscience/paranormal. OOBE's have been tested under scientific conditions. A popular test, is the "card test" the subject who is being studied has a card in front of them turned to it's back. Then the subject goes into the state of OOBE, and after it ends, the tester asks them what card it is was, and they can actually tell it.
OOBE and AP is a very well documented phenonema and many studies have been done on it. There are many people who have experienced them, and if you go to AP/OOBE forums there you will see people talking about the experiences they had, how they had them, and even planning meet ups between one another on the "astral plane" and reporting success. Indeed, it's not only these people who report this, but some scientists such as Monroe who have extensively studied the phenomena.
There have been many books written on it and the whole dynamics of AP been in great detail explained and methods on how to achieve them.
The only way of knowing whether all these people are having real experiences, would be to try it yourself - feeling is believing.