What would you consider to be evidence for God?

[Tonus]

In the history of man, the only thing we've managed to say with 100% certainty is that there is experiential stuff?

I get what you're saying.  And in the case of things outside of our ability to detect, I think it really comes down to whether or not they interact with the world we can experience.  If they do not, then we have no way of knowing if they are real.  In that case, all of the claims of such beings or planes of existence are made up, even if one of them might be accurate, since we have no way of experiencing them.

If they are able to interact with the world we can experience, then we must consider whether it makes sense that they can do so in ways that are consistent and give us a clear indication that (1)they are there and (2)who or what they are and what they want.  If not, we get a world where there are gods/angels/demons that we cannot possibly understand because the best we can get are inconsistent and fragmented experiences that lead us to varied and contradictory conclusions.  That could certainly explain many of the 'unexplained phenomena' that people report, while also explaining the many different varieties of religion and spirituality that people have (and continue to) practice(d).

But what most people seem to believe in (or want to believe in) is a universe with a specific and personable god capable of interacting with us in ways that we can experience.  And that we can share those experiences, whether it is by being with others who verify it, or telling others and comparing.  With enough of those experiences across the spectrum of humanity, it becomes easier to reach a consensus.  If a few people see a man walk on water, we are likely to want to know how he tricked us into seeing that.  If that man is able to repeat his feat regularly and in conditions where we can't duplicate or foil him, we begin to accept that he is doing just that.  Even if we can't explain it.

That latter may shake our sense of order, but we'd eventually accept that certain things are real, and that they are possible.  If the universe that is being claimed really does exist, then there's no reason we can't eventually reach such a level of certainty about things that we dismiss otherwise.