When I was in grad school, I worked in a lab that performed research on children with Asperger's syndrome (AS), mainly through fMRI and DTI brain imaging techniques. AS was merged into Autism Spectrum Disorders (ASD), but at the time was considered a high-functioning form of autism. I met dozens of children with AS; they were typically between 9-13 years old. All the children I met were clearly autistic. I'm not going to attempt to describe what that means here, but the nature of their disorder was evident, compared to other disorders and compared to the age-matched controls [1]. Back then I'd confidently tell you I could easily pick out the kids in a classroom with an AS diagnosis. These days, I have no confidence I could do so (mostly due to false negatives).

[1] anecdote: at the end of explaining the fMRI procedure to the participant children and their parents, I'd ask if the child had any questions. Neurotypical children would usually ask about any reward $ for completing the task. AS kids would usually ask something poignant about the experiment.