This seems like good advice. Amateur diagnosis of mental syndromes seems to happen a lot these days.

It raises an interesting question though. Every time narcissism / NPD comes up, the general advice seems to be that you should stay they hell away from these people. There seems to be plenty of evidence that they can wreak havoc on the lives of people around them, particularly anyone in an intimate relationship with them.

So it's arguably a good idea to learn how to "diagnose" these people, at least for self-preservation.