You know right from wrong doing something and subsequently benefiting or suffering. If you are intelligent enough, you will see from the results of your or other's actions what is wrong and what is right.

Like if a snake bites somebody and he dies, then you should know that it is not good to play with snakes. If you say that you won't believe it until you have experienced it for yourself, then you will die too. We learn from observation what is right and wrong. What gives us pleasures, more productivity and eternal happiness, we will usually call right. What gives us depression, suffering, competition, jealousy, temporary joy and hatred is usually wrong.

--Swami Radhanandaji