Currently we define ADHD almost entirely by signs, not symptoms.
This is distinct from other mental health diagnoses. To know whether someone is depressed, or bipolar, or narcissistic, you have to know what they're feeling, what they're thinking and what's motivating them. To do that you need to interview the patient directly in order to evaluate them.
You don’t need to sit down with President Trump personally to assess whether he interrupts people pervasively, frequently blurts out comments, or behaves in impulsive ways. These are all observable phenomena.
This is why I argue you can legitimately make an ADHD diagnosis for him without having interviewed him.