Hi Gannis-
There is no way yet discovered to absolutely, positively, and for certain, prove that a specific message came from a spirit, dead friend, a past lifetime recollection, or your subconscious. It's an experimental quagmire.
The way that most of us decide that we are getting useful information from "somewhere else" is that it makes sense, fits into reasonable expectations, and often brings information not otherwise available, and not expected. That still leaves room for doubt.
The other reason that many of us are convinced is that after accepting, at least tentatively, that there is "something out there", is that we have personal experiences that are lucid and deeply moving in some manner.
This is a sort of "Catch 22", in that you have to be open to have exotic experiences, which means willing to accept them if they arrive. However, by meditating, such as using Bruce's soul retrieval methods (a useful form of meditation), eventually we allow the expectations and attachments to relax away and some kind of experience arises. Whatever it is, generally it will be powerful, and will leave you feeling as if you have learned something and developed unexpected insights.
But - that's not proof. Even if your Great Aunt Gussie came through a told you where the family treasure is buried, it still is not proof - at least not in the scientific sense. However, if you can actually dig up the treasure, or otherwise apply the nformation received, it seems to be a fair bet that it is to some degree genuine. That's about as good as it gets from this side.
However, if you'll wait for fifty or a hundred years you'll get the answer from the other side.
See you there-
dave