From the What Makes N's Tick blog (it's a great book, by the way)...
Since normal people play the same mental games narcissists do, unknowing what they know, deluding themselves, and reasoning backwards, how can you tell when you are dealing with a pathological personality?
I'll never forget the moment I realized the significant difference between people you should just shake your head at and those you should be wary of.
It was a little thing, a man I had known for years in my place of work, a man I had previously liked and considered a friend. But the moment resonated. I found myself stunned and gaping at him, as if seeing him for the first time, while that little voice in my head said, "He's shameless! Things have gotten so bad 'around here,' and he is so spineless that he's gone all the way to shameless!"
Reeking of false pity for "the old guy" he said he "didn't want to get into trouble," this viper said, "I don't want to sound, but ..." and then proceeded to BE what he claimed he didn't even wish to SEEM like.
But that wasn't the worst part. The worst part was that his act stunk. It was so transparent that there was nothing to see through anymore. The guy was as naked as Adam and Eve but too shameless to put a fig leaf on it like they did.
That alarm proved to be well-founded. The man was capable of anything and had been a troublemaker who liked to come between others all along.
But what I first noticed was his shamelessness. Like the Emperor in His New Clothes, this guy just expected me to refrain from calling him on this bullshit. In other words, this guy didn't care that I saw him naked: he just got in my face with it to make me pretend I didn't. (On the twisted premise that a thing ain't wrong if you don't get caught and called on it.)
Note how similar this is to a narcissist's pathological lying, when they tell you something they know you know isn't true. They don't want to make you believe it - they just want to cram this lie down your throat = make you accept it in silence as though it's true (to avoid conflict).
At that moment I sensed that a crucial moral boundary had been passed and that the man was depraved. The future proved it. To my shock, he proved capable of things I'd never have believed of him.
Since then, this tenet to watch out for the bad sign of shamelessness has always proved true in my experience.
Give me the man or woman who, caught with their pants down, is ashamed. Beware the man or woman who, caught with their pants down, flashes you the moon.