I don't know. Anything would do. In the simple context of a Christian God, he could just reveal himself instead of leaving room for doubt. Maybe show in a double-blind study that prayer to this God actually works. Mind you I don't think that would be necessarily conclusive.
In my more expansive view, I don't have the foggiest idea. Laws of nature being violated repeatedly on order? Something else that couldn't possibly have a natural explanation.
I do know that there's a price of a million dollars for anyone who can prove anything supernatural. The only condition being that the people who give the price set up the conditions of the experiment.
en.wikipedia.org