I can often tell, either by observing, direct interacting, or even just talking to people online. I'm very good at picking up on it. There was one guy at work who obviously had it. He moved and stimmed the exact same ways I did. I stim heavily, and it's very obvious. I never did get to talk to him about it, though; he had a different role in the company, and I only lasted in the job I had a month (it was aspie hell).
What blew my mind, though, was when a non-aspie I run with in an MMO I play picked up on me. I'm not really that obvious, though I'd been playing with this guy for quite a while. We were discussing various things at one point and he asked me if I had any diagnoses. I told him I wasn't diagnosed, but was completely certain I had Asperger's. He told me he was just about to ask about that. I'm still surprised he picked up on it.
I sometimes wonder if a friend I had in high school has it. I'm pretty sure he does. He was always awkward, quiet, and very naive, but he seemed drawn to me, and trusted my judgement. He also has the "look", if you will. It's been a few years since I've really talked to him, and I last heard from him back in January, but I know he's still local and he still has my contact info. Maybe some day I'll get to talk to him about it.
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Heart of the guardian, way of the warden, path of the exile.