Insanity is not subtle, Bruce Charlton explains:
I spent a year in the 1980s working as a psychiatrist participating in the admissions rota where I would cover all the medical work necessary in a large hospital overnight or at weekends.
Quite a few of the patients were brought in by the police, by ordinary police officers — who had been called to some incident and recognized that the person involved was ‘mad not bad’, and so brought them in for psychiatric evaluation instead of putting them into the cells.
The police were never wrong, in my experience. The people they brought in were always crazy — it was just a matter of sorting out what kind of crazy. In other words, an ordinary policeman was able to tell when somebody was insane — it was a matter of common sense (plus relevant experience).