What godly miracle would it take?

[drfuzzy]

A personal appearance would be nice.

And then testable "miracles".

Hell, I might be convinced there's something in it if a faith healer truly carried out a measurable cure.

Yep, you beat me to it.      So, this GOD shows up at a TV station.  In the flesh.  Morphs into a few different looks on camera, then causes all amputated limbs to re-grow instantly, all over the world.  Heck, maybe it just heals everybody, everywhere.  Then it goes through physical examinations and lots of interviews on camera.  It explains where it comes from and whether any of the human religions come close to being right.  

Then it would likely turn into some sort of overlord that would probably need killing.