The telltale sign of autism is unbalanced growth. A neurotypical child grows at relatively the same rate with regards to their intelligence, their social skills, their hygiene, and everything else. Autistic children will usually show much larger growth in certain areas and weak growth in others. The reason it's more difficult to diagnose adults is that we've had enough time to balance things out by developing coping mechanisms for ourselves.

So while I've always been a very fast thinker, it took me muuuuch longer to develop emotional intelligence. Each of us has our own brand of autism, so forget any preconceived ideas about what you think most autistic people have in common. As is often said, if you think you're probably autistic, then you really are probably autistic.