Post History

This question seems to implicitly (at least) confuse behavior and diagnosis. Behavior is how the person acts, and novels are all about behavior, no matter which narrative POV is chosen. Diagnosis is what happens in a psychiatrist's office. Unless your character goes to a psychiatrist and gets diagnosed, putting a name to the behavior is beside the point. And if the character does go to a psychiatrist and get diagnosed, that event becomes important, and probably central, to the plot. 

If this does not happen, then the only way that the diagnosis of sociopathy, or any other psychiatric disorder, occurs is if either:

1. The reader does the diagnosis, based on the behavior they observe. Some readers like to do this. It gives them a sense of superiority and knowingness that helps keep them warm at night. 

2. The author at some point just says, "Hey, reader, in case you haven't figured it out yet, she's a sociopath." And what, exactly, is the point of that? If it is a surprise, then you haven't told the story very well. If it is not a surprise, then what's the point of saying it?

Personally, I would think it very difficult to write a book with a sociopathic protagonist. I find it hard to figure out how a sociopath can have a conventional character arc. How can they have a mirror moment, an inmost cave, when they ask themselves the great man/mouse question? There are no moral dilemmas for them, only operational problems. Sociopaths make great villains, but lousy protagonists.