What would you consider to be evidence for God?

[Psychonaut]

Isn't it really that we've just figured out what works, from what is observed?

From what is observed, and what is tested, and what is confirmed.  We know that radio waves exist because we see the results attained by those who did more than just observe and theorize.  We know what cholesterol is and how it works because of decades of medical and scientific research and testing.  We know that aliens routinely visit our planet in their little space ships because... well, many people don't believe that at all, in spite of all of the evidence and claims.  Why?  Because we have ways to learn and to verify and to add or change to our constantly growing collective base of knowledge.  This allows us to accept some wild claims if they can be proven (do you know that you can hold the contents of the Library of Congress in a device the size of your thumb?) and it allows us to remain suspicious of other claims until they are proven (Bigfoot, as an example).

The question of "what would god need to do for you to believe he exists" often relies on a single experience or event.  But we already know that such occurrences do not convince most people.  We know that they occur all the time to people from various walks of life and varying belief systems, and that they are used to confirm or deny all kinds of beliefs.  We don't accept those claims as true because our experiences through human history have helped us to develop ways to determine which are reliable and which are not.  If god wants to convince us that he's there, he just needs to show up and let us do the rest.  We've gotten pretty good at figuring this stuff out.

I agree that having a testable, repeatable, falsifiable hypothesis is of primary importance when seeking what works.
in the 5000 years of civilization we've had, we still haven't figured out a solid method for determining the "truthiness" of a claim. Other than descartes' cogito ergo sum, which would probably be better said as "there is experience, therefore verification of experience is immediately possible" as concepts of "me" and "am" are vague and crazy.
In the history of man, the only thing we've managed to say with 100% certainty is that there is experiential stuff? 

That doesn't sound much like progress to me. I know, planes, space, medicine, etc. It just seems that this is fairly important water we're wading through.
One would think we could've moved slightly forward in that pursuit (if it's even possible to do so).