Dave
that is an outstanding question and one that even the specialist are still grasping at.   I deal with severe cases in children and I am not an expert by any means just someone who cares and does what he can to help. The autism spectrum disorder has been expanded in recent years. Where the line is drawn I don't know. ADD, Panic Attack disorders, eating disorders can also be associated with autism.. sure but they are disorders in themselves. In children there are a number of early warnings signs for severe autism .. lack of eye contact, repetition, hearing issues (identifying sounds etc)... In high level autism (Aspergers) some of the signs most common are

Signs and symptoms of Asperger's syndrome include:

    * Engaging in one-sided, long-winded conversations, without noticing if the listener is listening or trying to change the subject
    * Displaying unusual nonverbal communication, such as lack of eye contact, few facial expressions, or awkward body postures and gestures
    * Showing an intense obsession with one or two specific, narrow subjects, such as baseball statistics, train schedules, weather or snakes
    * Appearing not to understand, empathize with or be sensitive to others' feelings
    * Having a hard time "reading" other people or understanding humor
    * Speaking in a voice that is monotonous, rigid or unusually fast
    * Moving clumsily, with poor coordination
    * Having an odd posture or a rigid gait

(taken from the aspergers site on the web)

One thing that I have notice that seems to also be present across all of the autism spectrum is associated seizures) only a doctor with some brain scans and study can tell if it is a form of autism or one of the other problems that need help with.