You know you're an atheist when you only thank your front line when you score a touchdown.
When you are singing but avoid the religious words!.
...when you refuse to trade reason for utter madness.
There was a pretty funny one I saw somewhere else "You know you're and Atheist when "god" is something you say on your back, not your knees."
When him/her get a hold of a religious book the first thing they do it's to write: WARNING THIS BOOK IS A WORK OF FICTION, everywhere possible.
You know your an atheist when hear little difference between PRAY and PREY.
You know your an atheist when you use the word Jebus as an exclamation.
You know you're an atheist when satisfying sexual desires does not make you feel guilty.