People on the spectrum are diagnosed by three criteria:
1. Communication difficulty
--Not using language, or using it in non-communicative ways
--May not be present in Aspies
2. Social difficulty
--Not understanding tone-of-voice, nonverbal language, etc.
--Difficulty with gestures, eye contact, volume control
3. Limited, repetitive behaviors
--Stimming
--Special interests
However, there are a lot of other things that are closely associated with, but not diagnostic of, PDDs. The uneven IQ distribution between types of intelligence is one of them; so are savant skills, sensory integration problems, asexuality, mental retardation, seizures, and various sorts of genius or special aptitude (which would be savant skills in a person with low IQ).