"The reality is a narcissist will rarely be diagnosed" absolutely Sago because it's never their fault, because they are always the innocent victim, they're extremely unlikely to ever seek professional help.
If they do, it's likely that the behaviour they're describing in their tormentor is fact their own, and the affect that behaviour is supposedly having on them, is what their victim is going through.
There's a lot of information available about all manner of psychological problems so it's very easy to find out about narcissism in general and the traits associated with this personality disorder.
We believe our ES's wife to be a narc based on what we've read, what we've been told about her past, from our own personal experiences and what little we know of how she is and behaves in the present.
I can understand why some may think 'how convenient, you can absolve yourself from any responsibility and blame your d.i.l.'; these things are said.
Whilst it is true that only a trained professional can give a definitive diagnosis that should not totally detract from personal experience.