Depending what type of atheist one is asking, this question may or may not even make sense.

If one is asking a strong atheist - one who positively believes/asserts that there definitely is no god - most atheists would be right there beside you, wondering the same thing. Though, it depends heavily on the god definition. Definitions that are too narrow/specific are easy to disprove.

In short, the authors of this FAQ believe strong atheism is not justified.

On the other hand, when asking a default/weak atheist the question, the question does not make sense. The default/weak atheist position is "You haven't supplied sufficient evidence to convince me that your claims are true." (since the theist has the burden of proof).

... so you'd literally be asking the atheist what evidence he/she has that proves that "you haven't convinced me" is true and accurate, since default atheism is not a position that no gods exist.

Asking such a question would be like asking "what temperature does the number 7 melt?" It is technically a question, because it has a question-mark at the end, but the question itself is gibberish, when applied to numbers.

Before asking the atheist the question, you'd benefit from clarifying whether he/she is a strong or default atheist, and go from there. Asking this question to default atheists makes you come across as some unthinking fool who does not understand the topic sufficiently to even ask a coherent question.