You raise some good points, especially that it's important not to react with a disproportionate amount of alarm when a child is not 'right on target' in some areas of development. Children develop at various rates in the different domains of development - this is not unusual. In fact, it's the norm. It is also not the norm for children to be able to read and/or write before the fist grade, even though some can do so. Because of the prevalence of pre-school experiences during the past decade or two, children often do develop some 'academic' skills earlier than used to be the case. However, there should be no onus for them to do so, and we needn't view our children as somehow deficient if they are not reading at kindergarten age. Relative to autism-spectrum disorders, speech delay in itself is not indicative of autism. What would be of concern is a child's not displaying the capacity to relate. It's the relational inability that is the foremost deficiency among children with autism and disorders along the Pervasive Developmental Disorder spectrum. The most reliable gauge of symtoms in this regard is the DSM-IV; it is the standard by which such diagnoses are made. Checklists and developmental screenings are useful in pinpointing symptoms, but they are not to be used to establish diagnoses. They can be a useful component of a diagnostic process, but should not be a substitute for a thorough assessment.
To a casual observer, he may seem active but a little odd in some respects. When most people unfamiliar with autism think of the disorder, they have the most severe of its form in their minds, not higher functioning autistics. My son is only 3 years old now so we don't really know how his ASD will manifest itself as he gets older, it could worsen or it could get better. There is more, but I hope this helps.
He was a late talker, but we were not worried, and by age 4 or five, he was all caught up, in two languages. Going back to his baby journal, I can now pick out his preoccupations, that are the hallmark of autism, such as spinning objects (while other kids enjoy watching TV, he enjoyed/still does watching the washer go around and around.
But the diagnosis became official when his difficulties of social interactions could not be covered by ADHD alone any longer, which was by 5th grade. I was not shocked. I knew about Asperger's then, and suspected my husband to have it. It was a relief, actually, to get yet another label for the kid, because that granted us access to the resources we needed for him to succeed (IEP, healthcare, social support and occupational therapy outside school.
So, at what point does behavior become autism? I believe, regardless of all the regression talk out there, that children are born with the disorder. A parent will "know" that their child is different on some level, because these infants are harder to parent, in my experience.