Question: How do they know it is real? What tells the experiencer that their Near Death Experience is real instead of just a dream, etc.?
Answer: You know it is real, because you remember the spiritual world from having been there before. We have all been in the Greater Reality of Spirit and know what it is like. We enter the physical through the spiritual. This is very clear from the knowledge gained in the experience.
All of us know what dreams are, having experienced them thousands of times, a near death experience is certainly not a dream. Neither is it an hallucination of a dying brain. Why? Because a NDE contains information unknown to the experiencer from their physical existance. It would be impossible for the brain to have information of this nature.
Many experiencers mention "being home" or "remembering the light" or "remembering that they are spiritual" or "knowing about the place they are experiencing." Most are comfortable and feel so good they don't want to return.
So, in summary, the answer is simple. They know it is real because they have experienced it before.