Good question; I think diagnosing it is almost a process of cross-referencing all the different symptoms to see if they correlate into the person being close enough to be classified asperger's, or less so but still PDD-NOS, or the other way into "classic" autism. Not that I know what I'm talking about, this is just my impression.
It's not perfect, and maybe in the future we'll have more tested and concise diagnoses and tests, but for now, I'll take it.