I'm not sure what a deity is, exactly. Could you give me a close definition?
I'm not really sure either. Pretty much every theist you talk to has a slightly different definition, and they're usually pretty fuzzy.
But the general description is something along the lines of an all-powerful being existing outside the bounds of natural realm.
Guess to a certain extent we just have to take it on a case by case basis.
All I can say really is that I have never encountered a description of a deity that I believe to be true. If I ever run into one, I'll stop being an atheist.
Of course someone could point to a tree and say: this is my god. Then I'd be inclined to believe that their god exists, but we gotta draw the line somewhere.
Also, to my way of thinking, belief is not a Yes/No question. I don't either believe or not believe. I assign rough percentages to the truth value of things and my percentages of belief change daily as I get more information or clarification.
The way I generally define belief is a nearly absolute measure of certainty. When I consider something to be proven to me, beyond reasonable doubt, I believe it, otherwise I don't.
It's just semantics really, I just always find "belief" to be such a strong statement that I avoid using it about anything I'm not highly certain about.