Do you think it's possible for a psychologist or psychiatrist to diagnose Trump with a mental illness just from observing him?

I’ve kind of stayed neutral on that point until I read the following from this article:
” That argument [that they have to have him in person]) has always struck me as nuts. There is no diagnostic blood test or brain scan for narcissistic personality disorder; there’s just a list of observable traits.

I have no comment on the source. I have a feeling it’s not the greatest, but that comment struck a chord with me. What do you think?