What is it about dogs that they sense good or bad in someone?

They don't see the facade, they just see what they see, if you know what I mean. I think that they can "smell" or sense bad in people, I really do. I had a dog, God rest her beautiful soul, who was the most gentle dog you could ever know, but once in a bloom moon, she would growl at a person, that low, warning growl that says, "Stay away, or else." I really feel she knew if someone was not a nice person or seemed like a threat to her.

I don't know how they know. But I can tell you I do NOT like anyone who does not like dogs? And anyone my dogs don't like, I have nothing more to do with that person. Dogs DO know good people from bad. I have seen it work and they seem to be accurate.

I do believe dogs have some special intuition, hence they are able to sense any thing that is not the norm ahead of human beings. Whenever I heard a dog growling, my antenna would be on alert. I keep no pets though.

Yes, I have a Peke like that. I even call him my best judge of character...lol. Believe it or not, I have a hard time trusting anyone he does not like. He has been on the mark a few times when I was on the fence so if he growls I get weary. He barks at everyone, but growls at only a few. He's a good boy who warms up to most people quickly. I don't know how they do it, but they always seem to know. We even joke with my daughter that when she is old enough before she will be able to go out on a date he must come into the house. She said "Oh, you want to meet them?" I told her no, I want Sparky (my Peke) to meet them...lol.