It's tricky, because part of Narcissistic Personality Disorder is the genuinely held belief that you are absolutely fine. If their behaviour is ever shown to be unpleasant, it can cause a massive meltdown (and then they'll often conveniently forget.) That's part of the issue - needing that safety net of belief that you are absolutely perfect, and anyone who doesn't think so is obviously wrong.
It's difficult, because the opening post feels inflammatory. I have a 'toxic' father. He has NPD. I spent a long, long time dancing to his tune until it became dangerous for me, and even then I feel panicked and want to go back to the time when I was carefully stroking his sensitive ego and protecting him from all the ills of the world.
Do I think my behaviour has always been perfect? Hell no.
The thing is, if you live every day in that state of panic and guilt because someone close to you has NPD, sometimes the extreme reaction of cutting contact, not even bothering to buy a solitary birthday card, sometimes all of that is simply the most sensible thing to do.
From his point of view, he could probably pop up and say 'My DD won't see my dying mother! What sort of cold, hard-hearted cow did I raise? She couldn't be arsed to buy me a birthday card this year!'
But he'd probably also come out with 'and she won't even let me see my grandchildren.'
That, for me, would be the thing that would make me think 'NPD'. For a parent to go out of their way to ensure no contact between their child and another - something has usually gone very, very wrong. It's not a light decision. For someone to feel they have a 'right' to someone else's children - that for me says an awful lot about a person.
Sorry - didn't answer your question. I don't think anyone is perfect, but if you've lived with NPD for years and years, sometimes the things you do might feel extreme to people not within that situation.