Tom's offered method of verifying for yourself is meditation. Expansion of consciousness to experience "non-physical" states (non-physical is in quotes because any primary reality frame appears to be physical to the entity experiencing it...just as any entity outside our physical reality experiencing our reality would have the view of it being non-physical from their local reference point), and exploration of those states is how he came to his conclusions.
It takes time, hard work, and dedication to verify these things on a personal subjective level, which is the only level at which you can really evaluate this kind of thing.
As a point of interest, Thomas Campbell was one of the scientists that helped Robert Monroe set up his experiments. He works as a nuclear physicist now and worked on missile defense systems for the USA.
