You may be interested in a This American Life episode called The Psychopath Test. It discusses the PCL-R (Psychopathy Check List—Revised) and how it's used in criminal justice systems. For many years, the creator of the test - which was incredibly accurate in predicting likelihood of reoffending - refused to share it with law enforcement. When it was finally published officially, and started being given to prisoners, it was considered objective science and became used as a reason to not give people parole:

The political reality, Carbone says, is this. In California, not only the board, but the governor must sign off on every parole granted. And there's just no benefit to being seen as soft on crime. So if a psychologist gives a bad report because of a high score on the psychopath test, even if the board or the governor wanted to set that person free, there is, as Carbone points out, no political cover if the prisoner re-offends.

It tells the story of one specific prisoner who scored high on the test. It's a very interesting episode.

-How is this measured? What's callous to one person may seem perfectly reasonable to another.

Ahhh, but Grasshopper - you write this from a lay perspective, that of one looking for wiggle room. A clinical diagnosis of Antisocial Personality Disorder (behavioral sociopathy, as opposed to personality sociopathy or psychopathy) is performed by trained, credentialed professionals who can expect their evaluations to be peer-reviewed. And, as criminal and civil law are tied into DSM/ICD10 definitions, "callous unconcern" becomes a legal definition, one subject to full legal review. As such, it isn't a "is he or isn't he" discussion, it's a "what stands up in court" discussion which varies state to state, city to city, country to country, etc. Eight months ago I served on that rarest of legal birds - I was foreman for a mental health jury. There are only about 200 in the world, and the one I was at - the only one in the world that allows for juries, they said - isn't in session more than a couple times a year. Me and eleven other people had to discuss whether or not the defendent was "grossly incapacitated" as defined by two sentences provided us in our jury instructions.

"How is this measured?"

Extensively.

Do you know any diagnosed sociopaths, or anyone you suspect of being one?

We were sitting down to dinner with Bob and Carol (not their names) for what was always an interesting, pyrotechnic meal. The appetizers were late and Bob and Carol were sniping at each other. At one point Carol said "well, my shrink back then said I was a sociopath but what the fuck does she know?" My wife and I mutually heard Ally McBeal record scratches across reality, but the meal continued unabated. My wife knew Carol because they were both in school together; when Bob came to me six months later to ask for marital advice I advised him to divorce her. We lost touch with Carol but Bob is still a friend. He's happily married with two kids in Maryland. So far as I know, Carol had no criminal history outside of a couple drug busts and charges of prostitution, which were dropped.

Of all the people I know or have known, three of them are killers. The only one I'm still Facebook friends with had extenuating circumstances; the deal went bad and they ended up chasing him across the desert in a pickup truck and shooting at him. He managed to get out of it by firing a 44 magnum through his rear window and hitting the driver in the face. The other two are bad dudes.