Thanks, glad it's of interest.

In this case, Harry was narcissistic before he learned about magic, when he had no realistic chance of boundless success (when he was one of many child prodigies, most of whom would turn out "pretty much ordinary" (Ch. 10), not the only magician-scientist), which is evidence that Harry's narcissism was due to his upbringing, not due to a realistic awareness of his own strengths.

I disagree with the premise that off-the-scale smart people are usually narcissistic, but I agree that many child prodigies are narcissistic. The work of doing research or another off-the-scale smart person activity encourages humility because of repeated failures (incorrect theories, etc.) on the way to new successes. Child prodigies (especially with a narcissistic parent, who distorts results to protect their own ego) can seem to go from success to success without apparent failures and while feeling fundamentally superior to others.

The way you tell is looking for narcisistuc traits other than "expects respect", for instance belittling others.

How do you tell if someone off-the-scale smart is unreasonably narcissistic or simply aware of their own strengths (and weaknesses)?

You compare their behaviour and the way they live their life to that of garden variety narcissists, and to that of most very intelligent people, and you see what they look more like.

Narcissism is a complex behavioural pattern with many very odd traits.

A track record of achievement or lack thereof is the obvious answer. Much harder to apply to a child, of course.