We know something is crooked when we see it - even though we’ve never seen something perfectly straight. We are comparing it to an idea we have in our mind about straightness. Your conscience may be evidence of the existence of “moral straightness” but that does not tell me why I should follow its edicts.
You may slap actions with labels like “good” or “evil” but your conscience, by itself, offers no more than your preferences. The real question is, “How do you move the beyond your opinions as more than your preferences? How do you move into the realm of an ethical "ought"? And you’ll want to get to the “oughts”. Because otherwise, you have no way to condemn the mass murderer who claims he didn’t know any better or has another sets of moral views than you or simply comes from a society with a different belief system than you do.
Perhaps a whole lot of consciences saying the same thing will do it. If society has general rules, is that enough?
It could very well be that your conscience is just your "superego" at work, blandly repeating the rules society has taught you. Leaving you operating out of a sense of guilt.
“So, how do we know what is right, given that “good people” disagree on a number of moral issues?”
While religion is filled with superstition and hypocrisy, there are plenty of exceptions and pointing at people’s inconsistent behavior is not a substitute for addressing the issues. It’s not reasonable to argue that “because the politicians I know behave in a blameworthy manner” therefore “all politicians act immorally.”
What if a society decides it's ok to kill Jews or some other minority? How do we justify taking our society's views and imposing them on others (like Nazi Germany)? Somehow these "rules" must supersede individual societies, so they apply across cultures and generations.
Stephen Goforth