Let's say I pass you a twenty dollar bill. How do you tell if it is real or not?
You could attempt to study every counterfeit variation or mistake ever produced. Quite a task.
A much easier way would be to study a real twenty dollar bill in great detail; one could then easily see what is real and what is not.
How do I know if my supposedly real reference sample for a $20 is a real $20? Perhaps I'm comparing a fake to a fake.
I would just ask the Treasury Department if it's real and get an absolutely definitive answer. In the case of deity, I can go to a hundred different supposedly authoritative sources and get a hundred different answers. Most will tell me their $20 is the only real one, leaving me broke if I believe them since I only have a wad of $20s verified as genuine by other authorities.