Essentially, we'll never truly be able to distinguish between "right" and "wrong" actions. At any given time in history, however, philosophers, theologians, and politicians will claim to have discovered the best way to evaluate human actions and establish the most righteous code of conduct. But it's never that easy. Life is far too messy and complicated for there to be anything like a universal morality or an absolutist ethics. The Golden Rule is great (the idea that you should treat others as you would like them to treat you),
For example, should the few be spared to save the many? Who has more moral worth: a human baby or a full-grown great ape? 

At best, we can only say that morality is normative, while acknowledging that our sense of right and wrong will change over time.