No, I can't tell just by looking at someone, unless they are rocking or stimming, then the thought will occur to me. I can sometimes pick out traits in people who I've known for a while, or in a student, and if I know something of their history or that they are having trouble fitting in at school, I will suspect it, but unless the person tells me "I've been diagnosed with AS" then I really can't know for sure. Only a trained and qualified specialist can make that judgement for sure. I've realised that a couple of friends I had at Uni may be on the spectrum, one I'm 90% certain about. About 3 of my past students had strong autistic traits, but I don't know if they were diagnosed. One current friend has strong ASD/OCD traits - again, I don't know if he's been diagnosed with any condition. I doubt it, he doesn't like doctors.
A couple of years back, I had a student who showed a couple of traits - extremely intelligent, reading way beyond his age, high IQ score, difficulty staying on focus. At some point he started becoming very unhappy at school, and he started acting up. He also wasn't getting on too well with his classmates, and he had an obsession with zombies, difficulty with handwriting. His mum mentioned Aspergers and ADHD to me, and I said yes, he does have a couple of triats, I sent her some info which she read, and said that some applied to her son, and some didn't. I said, well, just to be sure you could take him to a specialist and get an opinion. So he saw a specialist, and it turned out that he was gifted, had a couple of ADHD traits, but not ASD. The specialist said that the reason that he was acting up was because he was unhappy at school, and gifted kids often have this kind of problem. Gifted kids can often find it hard to fit in at school, too, and it can look like ASD. I'm glad to say that he has changed schools, jumped ahead a year and is now doing much better. Those 'traits' have now mostly disappeared.
I guess the point I'm making is that other conditions can produce ASD-like symptoms, and without a more detailed investigation it's easy to mistake symptoms of other conditions with ASD - just because a person shows traits it doesn't mean that they have it.