When people ask me if I believe there's a God, I usually answer by saying:
Well, there's a long answer and a short answer.
The short answer is "No."
The long answer is "Nooooooooooooooooooo!"
No. Nor do I believe in Santa Claus, the Easter Bunny or the Great Pumpkin!
You could tell them you're an Igtheist and see how they handle it. lol
Here's the definition if they've never heard of it.
To the ignostic, unless a theist can provide a concrete, detailed and falsifiable definition of what they mean by "God" then there is really no point in discussing deeper issues involved with "God" such as whether or not it exists.