You have to be pretty bigoted to claim to know exactly where good ends and evil begins. Shakespeare recognized "There is neither good nor bad but thinking makes it so." This is the human condition! So how can we discern what we need to.
An interesting object-lesson here is Sir Winston Churchill. He knew what Hilter was long before virtually all his peers. Many, including the vaste majority of "religionists!" were earnestly trying give Hitler the benefit of the doubt, and even in the midst of horrendous rumors, trying to appease the man! When Churchill was finally "given the reins," he defied popular opinion and declared he would never negotiate with "that man [Hitler]." He went on to lead Great Britian against "impossible" odds to final victory. Similarly, he knew what Stalin was when others, including Roosevelt, were so naive, charmed by Stalin's personality.
Why was Churchill so right and the others so wrong? We attribute it to his spiritual sense, that which we all possess, but we mainly ignore. He smoked cigars and drank liberally, was hardly a religious man in the usual sense. But he had a sense of Principle and was not swayed by opinion.
In the 1st Ed of Science and Health, there is a constant theme of Principle vs personal sense (see Bible Thread on this website) - understanding vs belief. Personal sense is a state of belief, Principle a state of understanding. Personal sense believes in right and wrong based on personal experience and perspective. It is thoroughly unreliable, and yet it governs the majority of people. Principle is based upon that which is outside of personal experience and perspective, often referred to as God (although I am not sure Churchill would use the term) - spiritual sense that sees right through the sheep's clothing to the wolf!
There is nothing more important for us, and our children, than to cultivate this spiritual sense, for without it we could be entertaining a wolf and not even know it!!