Well give me an example of how this would work in practice. You pick up your soul-phone (or get help from a medium) and the conversation proceeds:
Hi Mom, I'm so glad you are here, I have missed you so much!
Jane, I love you so much, though to be fair, you didn't come and see me very often while I was alive!
Oh Mom, that is true, but it is so nice that you are here!
etc.
Now how should Jane or a neutral participant evaluate that for its loving content? Wouldn't most people want something more concrete to make sure they were talking to their mother?
A trickster could obviously fake the above, and if he had sufficient psychic powers, he could answer any test question. This is basically the same problem as the super-psi argument against mediums. If you assume enough psychically gifted evil entities, you can dismiss any evidence.
That doesn't mean that it isn't reasonable to ask a few test questions. For example, Jane might ask, "Do you remember my 11th birthday?" knowing that it was special in some way - she was in hospital, or lost in a national park, or received a particularly exciting present. The reply would be very informative, I think - but would that count as testing the spirit in question?
David