What would it take for you to believe in God?
Firstly I would have to ask which god? Yaweh? Allah? Zues? Vishnu? But getting past that and assuming they meant whichever god they believe in I suppose there are a lot of things. Perhaps something written in our DNA that has their signature in it? Like “Yaweh was here”. But readable in every known language. This would have to be in the DNA of every living thing.
I often thought that maybe magical sky writing they everyone could read in their own language but that could be alien in nature.
“Any sufficiently advanced technology is indistinguishable from magic.” Arthur C. Clark
The long and short of it though (borrowed from Matt Dillahunty) is that if there was a god, it would know exactly what evidence I would need.