Atheists do indeed know the difference between right and wrong. They know right from wrong exactly the same way religious people know it, they learn it. At an early age a child’s caregiver imparts lessons and examples to that child as to what is or is not acceptable behavior. When and where that child is raised has much to do with that child’s eventual perception of right and wrong. A child raised in a traditional American Christian home will come to know right from wrong based on the lessons learned from that specific perspective and will have those concepts confirmed as he moves around in a society where those tenets are considered the norm.
The child eventually takes these lessons and as he goes out into society learns the more subtle facets of right and wrong. What is acceptable in one place or time may not always be acceptable in another.
The Atheists and the Religious
What is different about right and wrong between atheists and the religious is not how that knowledge developed, but rather the specific list of things that are right and wrong.
In some religions, polygamous marriages, including marriage to minors is not wrong. In some religions alcohol consumption is completely forbidden, yet in others in varying degrees, it is not. In some religions sacrificing animals (and in past times even humans) is right, to not do so is/was wrong.
A female pastor? Is that right or wrong? Praying to Jesus? Chanting to Ganesh? It depends, does it not?
For many centuries, even in Christian cultures, slavery was okay, as was the subjugation of women, the elimination of heathens, the burning of witches, the list goes on. Christianity didn’t create morality, Christianity has instead had to adapt to an evolving set of social moral codes.
As much as some folks would like to claim otherwise, absolute, universal right and wrong do not exist. Right and wrong, morality, is indeed relative; therefore an individual’s sense of right and wrong is formed and dictated by the society in which that person resides and that person’s willingness to adhere to that society’s rule set.
Enforcing a code of conduct
Anarchy, the lack of moral codes and enforcement would not serve any society well and that society would not survive for very long. Only those cultures, societal units that establish at least a basic moral code will survive long enough to flourish and reproduce. This is why all religions have the same basic list of rules against murder, theft, assault, etc. It is because ALL successful cultures must have at least these on their list to merely survive.
Right and wrong are not owned or invented by religions, right and wrong are merely redefined by religions primarily to serve the needs of that religion.
Atheists certainly know right from wrong, they obtained that knowledge the same way religious people do, the old fashioned way, they learned it.