I had never heard of Asperger's or autism before my diagnosis, so I had no clue until my mom told me that I had it. What tipped my parents off was watching an episode of Dateline or some similar show that featured Asperger's syndrome and seeing a lot of similarities with me, I'm guessing probably just the well-known stuff like obsessive interests and communication issues. But there isn't really one thing for anyone that you can point at and say "See, this means you have autism." It's a combination of factors, and is as individual for each person as each person is from each other. It really isn't a clear-cut thing in many cases. I would say probably a key thing is a sense of "other"ness, because regardless of specifics, an autistic brain is fundamentally different from a non-autistic brain, but then again, I have this feeling even among others on the autism spectrum, and there are also other things that make one feel fundamentally different from the average person.
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Yet in my new wildness and freedom I almost welcome the bitterness of alienage. For although nepenthe has calmed me, I know always that I am an outsider; a stranger in this century and among those who are still men.
-H. P. Lovecraft, "The Outsider"
When you assume, it makes an a** out of u and me.