Yeah, I can definitely tell if someone may be on the spectrum. Sometimes it's physical cues, sometimes it's the way they speak or the manner in which they respond to interaction or answer questions. For example, I was watching a horror movie last night with a friend of a friend who I hadn't met before, and two other NTs.. this guy was a movie buff, I guess it was his special interest, and any time anyone would say anything about the main character, crack a joke, or make a double entendre (yeah, old horror movies tend to lend themselves to double entendres) he'd respond in the most literal way possible. Another time, I met a very quiet guy at college who was a computer genius.. everyone thought he was shy/weird and 17.. I thought he might have AS so I chatted with him, turns out he was had an "anxiety disorder," in his words, and was 22. Another girl I tried to get with was so awkward in bed I eventually gave up... she had all the cues, especially oversensitivity to stimuli. My roommate in sophomore year of college was a sure bet. Pokemon was his special interest and he spent whatever time he wasn't eating or in class playing video games. He was difficult to socialize with, often speaking at length about what interested him. Oh, and he rocked all the time.
Never did I talk with any of them about AS. Don't want to give myself away.