If the gray-haired gentleman is the Simulator of our universe, then presumably he could demonstrate this by allowing you to "view" the simulation, in particular your decomposing corpse, grieving loved ones &c. Also, he could further replay to you events from the past: events you remember vividly, and perhaps some historical ones too. And also, he could allow you to spectate on the ongoing simulation.
Of course, he would no doubt have to provide a considerable number of bits of information in order for the Afterlife hypothesis to become more likely than the alternatives such as dreaming, drugs, major brain malfunction, &c.
This also doesn't differentiate 'dead but sentient' from 'brain in a vat', and I think the prior for the latter is a bit higher than the prior for the former.
You unexpectedly find yourself sitting in a windowless room across from a gray-haired gentleman. You didn't wake up there; you were walking down the street and cut to camera two, a white windowless room with a table and two chairs. After a moment, the gentleman speaks:
"You are dead, killed instantly by a small meteorite. Incidentally," he smirks, "you have lost Pascal's Wager. You may 'cross-over' once you can accept that you are dead. I am here to help in that endeavor and can present any evidence you desire."
You, being a stone-cold rationalist, will only reach this conclusion on the basis of solid evidence. He, being extremely ethical, will neither present false evidence nor attempt to undermine your rationality. What can he do to convince you that you have died?
I suspect there is nothing he could say or do to convince you of this. Rather, for any sufficiently "final" definition of physical death, there's no way he can demonstrate that you have somehow come out the other side. That's my wager: there is no sound way to convince someone, even while in the afterlife, that there is such a thing; thus, we should never believe in an afterlife knowing that we could never accept it even if actually there.
Am I wrong? Has this been proposed before? Is there any thing which, while actually true, could never be demonstrated in this manner?
I think that, if correct, this may point to a special class of untruths. Sort of... Bayesian contradictions, things which could never be sufficiently demonstrated.
Naturally, lukeprog's earlier post has me thinking on religious lines.