One way to calculate this probability would be to count all the Gods we "know" of so far. (Allah, Thor,Zeus, Baal, etc) Let's say in human history this comes to 500 different Gods.
Now, are these all so different from each other that they are mutually exclusive? Do they allow for multiple Gods to exist? To simplify things this time around say yes and no to those questions.
Next, are there any ways of determining if anyone is more likely than any others? That is, do we have evidence for any one that is really different from any others? The answer seemt to be know.
And now let's assume (just for the argument) that one of them does exist. They are all equiprobable so the chances of any one of them being right is 0.002.
As you can see it is possible to come up with a range of such probabilities with this approach. I would say the range of p would vary from zero to a number under 0.001 when it was worked out more carefully.
And then we can work on the number of angels on that pin head. ROFL