One way to calculate a probability for god existing would be to take all the people who have personally 'experienced' god and all those who haven't. Of course, you'd have to compensate for 'experiencing' incompatible gods. If we assumed that everyone is truthful and of sound mind, we would reach a relatively large number.
Now, this makes not attempt to account for social indoctrination, so a more accurate method would be to find the number of beings not exposed to a social system that believe in god. Unfortunately, these beings are probably not in existance any where in the solar system, so we are likely to have trouble there. Perhaps if we could check the beliefs of deaf new borns (deaf to avoid audio transferral of beliefs while in the womb) we might have a rough estimate.
Of course, the Gnostic and (historic) Pagan view that god can only be known through knowing yourself requires consciousness and self-study, which can only occur in adults.
Unfortunately, it looks like belief structure cannot provide evidence for god, we can only base a probability on the likelyhood of specific gods existing (the Olympians who live on Mount Olympus for example are unlikely to exist, as they are not there when we look).