The ability to tell good from bad is a pretty basic skill for a human being, actually.
The term 'mumayyiz' is often referred to a toddler who is able to differentiate between good and bad, right and wrong. Grown ups are supposed to be able to do more than that.
Sufyan bin ‘Uyainah is right when he said, "The intelligent person is not one who merely knows what is good and what is bad. The intelligent person is one who, when he sees what is good, follows it, and when he sees evil, shuns it.”
Saturate yourself with good deeds, so that it wont leave any room for the bad ones. Remember that whoever tasted the sweetness of iman will always recognize the bitterness of sins.