As has been previously stated, sometimes AS is not obvious. This isn't always the case though. Even when it is, it is usually a combination of symptoms that give a person away, rather than just one.
I have been told that I am rather obvious in many ways. I obsess over my topics of interest and have been told that I even have a habit of linking other peoples conversations to them. Despite being able to tolerate people, I like to be alone quite a bit. I have peculiar speech patterns and social behaviors. On top of this there are many habits I display or have displayed. I have auditory and visual sensitivities. Being prone to meltdowns is also a problem.
I can also sometimes recognize when other people have it, too. There was a girl I had a class with who's interest was in wolves, had a monotone voice, the well known intense stare of someone with AS who is not used to making eye contact but is certainly trying and several other symptoms. She had a more severe case than I did.
I also had a classmate at another time who had similar speech issues to my own, had trouble grasping social behaviors and had deep interest in his topics of choice. We became close acquaintances and when I finally asked if he had AS and he said yes, I told him about this website.
Furthermore, when I here about the lives of some of the scientists in the fields I am studying, I can't help but almost be sure they have AS. One example I can think of is a mycologist from the late 1800s named Buller. When I mentioned him to a professor he told me that the man had spent most of his time working in the lab, wearing horse blinders as he examined the fungi he was interested in. He only interacted with a couple people and when he did, it would just be for a meal and then he would go back to work.
When the professor asked me if that was weird, my response was that it didn't. It sounded like a man who was extremely interested in his work but was sensitive to what was going on around him so wore horse blinders to help block out distractions (which I personally thought sounded like a marvelous idea). I could picture the dinners with those couple of people happening when he desired social interactions but found that the dinners themselves were enough to meet his needs for spending time with other people before he went back to work. I could picture that the people he had dinner with were probably those he felt the most comfortable around. To me, this sounded like a person who could very well have aspergers.