How can you break into a conversation about if they are unsure if there is a God or not?  How do you find out that they are an Atheist?

One effective method to ascertain someone's beliefs is to pose a question such as "Do you believe in God?"

If their immediate response is not "No!" ask this: "If you believe that there is no God, we will call this a zero (0), or if you are absolutely certain that there is a God, we will rate this a ten (10). How would you rate your belief on a scale of 0 to 10?"

If they answered below a 5, you could ask them, "Why didn't you rate it a zero?" If they gave you an answer greater than 5, then you could ask them, "Why didn't you rate it at 10?" These questions may give you some valuable insight and lead to follow-up discussions. 

If they declare they are an atheist (someone who absolutely does not believe in any god) how could you continue the conversation?  If you are stuck on this please see this article.