How did Cain and Abel know to offer sacrifices before the Law of Moses? Genesis 4 has always bothered me. The story of God accepting Abel's sacrifice and not Cain's has led me to wonder: How did they even know to sacrifice? Was this practice started by Adam? Or did God establish more of a law than just 'Don't eat of the tree of the knowledge of good and evil'? If God did establish further law then what was it and when did He establish it?
That's a very good question.  Clearly there are a lot of details missing from the earliest parts of the narrative.  Remember that Genesis is traditionally attributed to Moses, who led the Israelites out of Egypt and gave them the Law of Moses.  Storytellers tend to explain unfamiliar concepts and not waste time explaining familiar ones, so it's reasonable to infer that Adam and his family were under a commandment from God to offer sacrifices in a similar, if not identical, manner to the rules about sacrifices in the Law of Moses.  Beyond that, the Bible is unfortunately silent.