In Book Two, chapter 11 of his Discourses (free online version here), Epictetus describes what he takes to be the purpose of philosophy–to function as a ruler against which we measure our arguments–and explains how it comes about that, given that we all are born with a notion of good and evil, there are conflicting opinions about who is doing good and who is not.
It is interesting to read, in the first place, that Epictetus believes the ability to recognize right and wrong is innate. He writes:
But as to good and evil, and beautiful and ugly, and becoming and unbecoming, and happiness and misfortune, and proper and improper, and what we ought to do and what we ought not to do, whoever came into the world without having an innate idea of them?
He contrasts this with knowledge about geometry or music, which must be learned, and “for this reason those who do not know them, do not think that they know them.” The examples he uses are those of a right-triangle or a half-tone; no one seems to have an instinctive sense of what those things are. And yet we do have a gut feeling about what is right or wrong, despite never learning it per se. 
But how, if everyone has an innate sense of right and wrong, are there differences of opinion? As it turns out, the beginning of philosophy “to him at least who enters on it in the right way and by the door, is a consciousness of his own weakness and inability about necessary things.” We simply need help to ensure that our preconceptions about right and wrong adhere to the true right and wrong. Epictetus describes the need for a philosophy-ruler, whereby we can weigh and measure certain arguments that all might appear to be correct, “as we have discovered a balance in the determination of weights, and a carpenter’s rule in the case of straight and crooked things.” In this way we assess which of the arguments that seem right, actually are. Simply doing what seems right to one is not enough, because even a crazy person can do that:
Does the madman do any other things than the things as in which seem to him right? Is then this criterion for him also? It is not sufficient. Come then to something which is superior to seeming.
So now we get to the juicy part. How do we use this philosophy ruler? How do we figure out who’s right and who’s wrong? Crucially, how can we adjust our behavior so that we’re always right in the future? I wish Epictetus had gone into some more detail, but here are a few examples of things you should look for. His scribe has used the typical discourse device of describing Epictetus speaking with a nameless person, who then responds. They are speaking here about pleasure:
“Ought the good to be such a thing that it is fit that we have confidence in it?” (Yes.)
“And in which we ought to confide?” (Yes.)
“Is it fit to trust to anything which is insecure?” (No.)
And this is my favorite:
“Is it fit to be elated over what is good?” (Yes.)  When he then asks whether it is fit to be elated over pleasure, Epictetus doesn’t even wait for the response, but says that if his partner says yes, then Epictetus won’t find him “worthy even of the balance,” or our handy philosophy ruler.
We have confidence in goodness, we trust in it, it is secure, it brings us joy. We can always hold up our arguments to these rulers and see if we measure up. Will we be worthy of the balance? Is this what it means to enter philosophy “by the door,” and not, say, the window? Only a madman would do that, because it seems right to him alone.
 We learn at an early age that if we act a certain way, we will get in trouble. So then do we hew to good behavior because we wish to be good, or rather to avoid punishment? We also learn, incorrectly, that the good guy always wins in the end, because cartoons and movies drive that point home relentlessly. My five-year-old has unfortunately learned that the good guy does not always win (they teach them in kindergarten that Martin Luther King, Jr., was shot and killed, for example), and yet he always wants to be Hulk when we play-fight, and for me to be Ultron. If I say I want to be Hulk for a change (I want to say “Hulk smash”), he gets very annoyed. But does he have an innate understanding of and preference for goodness, or does he want to play the “cooler” character? I am still trying to figure out how to proceed. So far, the Golden Rule has been most useful to me as an aid in teaching right from wrong on a fundamental level.
 Anyone who criticizes Stoicism as not allowing for pleasure in life forgets that Stoics are truly excited about virtue, also sometimes rendered as “excellence of the soul” (which does sound more exciting).