What's fascinating is that if you don't ask the question directly, it's possible to demonstrate to them that they do believe in G-d. They are influenced by the society, the educational system and their friends to think that they don't believe in G-d. He doesn't need our prayers or our praises. Most people are able to appreciate the concept and accept it. All good discussions must start with a definition or one ends up going in circles.
Belief is a point on a continuum of knowledge from no knowledge to absolute knowledge.
The more evidence that you have, the stronger the belief. The Torah concept of G-d is that of Creator, Sustainer and Supervisor of the universe and all that is within it. People do not like restrictions in their lives. When a person says he's an atheist, you can point out to him that an atheist is one who has evidence that there is no G-d. Then you can clarify that an agnostic is one who has evidence that a person cannot know if there is a G-d.