This is kind of what I'm getting at.
I'm not talking about accepting or being suspicious of presented evidence -- cleric casts gate, god steps through, miracles performed. I'm meaning, let's not conflate our metagame knowledge of the D&D world with an NPC's knowledge of his world.
Do common commoners know for fact that ghosts exist?
That werewolves exist?
That dopplegangers exist?
Drow?
The above all do truly exist in the standard D&D game world, but we know this because we have the books -- purely metagame knowledge. What if we could see the rule book for our Real World, and we saw the stats for space alien Greys? Or saw the rule mechanic for Karma?
Bullgrit
I have a standing rule not to tackle philosphy sober but I'll bend it a little, what do you mean by know?
Does a virus exist in the real world? or if you are a microbiologist and have examined virus in an electron microscope or sequenced their DNA, some other scientific phenomena that is outside your field of expertise and for which you do not have to background to do the experiements or the background theory, neutrinos, charm quarks or as I noted above quantum entanglement.
If you are willing to accept such phenomena on the basis of peer reviewed literature from the scientific community then in a place such as the Forgotten Realms where anyone with the lack of caution to actually go looking for the gods can quite easily get got up in their machinactions and where high level wizards can become gods and/or have sex with them then gods exist.
Now, Ebberon for instance, is a different matter, there even though there are clerics no one knows for sure.
Now, if you are arguing that even Elminister cannot have been sure that Mystra was a god, then I think that you are getting into silly territory.
For any practicle purpose a God is a godlike being.
If in Star Trek Q decided that the people of the Federation would worship him or die (and no similar or equavilent being stopped him) then in pretty short order Q would be worshipped by the surivors and they would be pretty convinced that Q was a god.