We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
Everybody may have a devil inside them, but the people we term "psychopathic" or "sociopathic" are outliers on the bell curve of human behavior. Sociopaths have habits of exploitation, deceit, cheating, impulsiveness, manipulation, rationalization, and often of cruelty. Clever sociopaths are masters of disguise and of acting. There is a genetic component.
Clever sociopaths can put on a "mask of sanity" and, if careful enough, make successful lives in some careers.
Actually, boundaries of conscience can be measured from a very early age. If they are too wide, they can be narrowed through operant conditioning. The trick is to challenge the child with scenarios of increasing freedom. As he learns to behave responsibly with simple problems, he can be taught to act correctly with problems of increasing complexity. The process only works, of course, if the corrections are immediate and meaningful. It's a big part of drama therapy.
Unfortunately, therapists make money by being nebulous; and by charging as much as possible. They never give the parents any benchmarks.
OK, non-expert here. I suppose you can make determinations about a child when they are 3-4 but I'm just not sure you would always be right or even right half of the time. My experience is that it is at puberty that these demon's become more clear. And it is post teen that they become most dangerous (in most cases).
Another observation; I don't so much fear the adult who obviously has a mental health problem. What I fear is the adult who has a mental health problem and can hide it.
When I was a medical student I spent some time with a psychiatrist named George Harrington who used a method, that sounds like what you describe, to treat chronic schizophrenics before modern antipsychotics came along. One of his residents at UCLA wrote up his concepts in a book titled "Reality Therapy."