I am not an athiest. I am not fanatically religious, just enough to believe in an afterlife and in God, but I'm not an athiest. 

That being said, I find it very disturbing most of the time when religious persons ask this question, as it suggests that if they suddenly found themselves in a world where the existence of their God was disproved,  they would go about murdering and whatnot willy-nilly. If the theory of creationism was disproved without a hint of a doubt, and you lost faith, (For good measure) would you suddenly lose your sense of right and wrong?

We can say that athiests know right from wrong by what their parents teach them, (And other environmental factors) and by the laws their Government imposes. (The more sensible laws, anyway) But this might easily be the wrong answer. I don't want to wax philosophic, as the first reply did. Right and wrong are indeed ideas, but they are in most cases based entirely in common sense...

'Thou Shalt Not Kill.' Very sensible, if you want to ensure the survival and continuing superiority of the human race. 'Thou Shalt Not Steal.' Also sensible, if you want to avoid provoking someone into ignoring that first example I mentioned. The rules against touching dead animals help to curb the spread of disease. Etc...