What would suffice as proof for me?
If aliens landed on earth, and told me that they worshipped the Lord, and they had their own version of the bible, with all of the major teachings within, such as love your neighbour and so forth, that would convince me that religion is not merely a culturally confined phenomena. After all, would God leave an intelligent race to rot in hell merely because he did not tell them all about his religion?
Failing that, I would accept God coming down from heaven, and smiting some people in a clear and unambiguous way. He done that often enough in the Bible, why can he not do that now?
Otherwise, when I die, I will only believe when I am sent to Hell, which apparently is the destination of all people who do not believe in God, no matter what sort of life they have led.