People determine what is right based on their morals. Not everyone agrees on what that means for every situation but there are some widely-accepted ideas for how to make those decisions. By adulthood, it's generally something you'll have an intuitive sense for since moral reasoning begins developing early in life. Even if you're deciding (or being forced) to do immoral things, people tend to be capable of recognizing that what they're doing is wrong. One problem with religion that is identified in the picture above is that in the case of a conflict between a person's faith and their morality, religious influence may override their sense of right and wrong which can lead to conflicts, both internal and external. I don't know if that's what you're asking though.

Hopefully that wasn't too vague of a response, I'm not the most qualified person in the world to explain but there's no shortage of reading material on the subject. It's one of those things people have been talking and writing about more or less since the dawn of talking and writing.