My DS11 has Asperger's. He was diagnosed with PDD (pervasive developmental delay - an autistic spectrum disorder) when he was 5. By the time he was 8, the school psychologist noticed more and more Asperger's characteristics and we, along with several specialists, have agreed on that diagnosis.
I also work in a special ed. preschool classroom so I see lots of kids who are just on the verge of receiving a diagnosis. When they're around 2 or 3, you can usually tell if they're on the spectrum. As we work with them more and assess their language skills, etc., it becomes more apparent if there is an Asperger's component at work.
All kids with Asperger's are different so it's hard to compare symptoms and behaviors. I've seen a few girls with Asperger's who are very similar to my son. Usually the differentiating factor between Asperger's and other autistic spectrum disorders is language and verbal skills. We see 3 year olds in our class with very poor social skills, no eye contact, stimming behaviors, arm flapping, etc., but they know the entire alphabet and can read numbers up to 100. They may still have a language delay but tend to acquire and use language faster than other kids on the spectrum. When those characteristics emerge, we usually start thinking it's Asperger's.