Interesting question, ChilliWilly (I like the code name by the way, I think there was a cartoon character by that name if I'm not mistaken).
But I digress.... I was saying you pose an interesting question. I would start by saying those people who are truly 'good' do not "know" they are good, if you know what I mean. And people, like Hitler, say, whom we would all agree -- I hope -- was practically, effectively "evil," nevertheless, as you know, certainly do not see themselves as evil.
But am I saying that a lack of self-knowledge on the part of an "evil" person, in anyway negate the evil?
I assume, ChilliWilly, that you ask this question because it is something you want to know; and I also assume that no answer from "scripture," or anything like that will satisfy you -- which is as it should be; I too dislike "easy" answers.
So, is it at all possible to "know" what is good and what is evil? Speaking in naturalistic terms (I use the word 'naturalistic' loosely) I would actually say that "human nature" is basically "good" or at least evolutionarily functional. Human beings (or the creatures that would morph into human beings) could never have survived as a species without developing characteristics of cooperation and compassion.
I would say that which is "good" furthers compassion, cooperation, life (but I am no "pro-life" conservative), art, and beauty. I would say that which is evil runs counter to that, indeed threatens the survival of humankind and the rest of the world while we're at it.
I would say that nuclear weapons (no matter who has them) are evil because they threaten global annihilation, to give one example.