There was a discussion about this on another message board I go on. Most people will display some symptoms, but people who actually have NPD are far more extreme than you can imagine. Some people there's parents had it, and it was hard for them to live with.
It is hard to diagnose, as people with it think they're right so refuse to go to a doctor or anywhere. They can't see there is any other way of living, and do not acknowledge anyone else's point of view. They can't even say something like "i see that but think X is right", they just dismiss it as nonsense and ignore it.
I personally think that someone I know has got it, but I can't say anything because I know I will suffer if I do. So it's easier to just ignore her.