Yeah, I was a bit perplexed about that on this board, too, thinking, wtf, there's that many people with Aspergers? Really? There can't be that many... I've only met 2 diagnosed Aspies in entire my life, and I've met one other diagnosed NVLD like me, too.
As far as detecting it, I mean, it depends, I can't read minds, and that's what you gotta do. My friend with Aspergers, we tend to sorta "think" the same way. Sorta the way I analyze it is the person's thought process. We're both influenced by logic and analysis a lot. We also tend to come up with different conclusions, and will argue all the time, but almost will never take it personally. We used to get into 2-3 hour long debates about random things like how airsoft guns work or something (which I won.)
However, I'm also almost certain this girl I know at the skating rink has NVLD like me. She seems almost exactly like me. We both go there during public sessions and just practice by ourselves. This is especially odd for a girl, as I live in a house full of girls, and my sister for example, she refuses to go out anywhere without a friend. She said she'd go ice skating, but only if her friend went. A lot of people I know are the same way, especially at my church, where it seems full of people like that, that just want to do things "together" with people, in a group. This girl here, exactly like me, just randomly went to my rink, liked it, bought some skates, and just practices. No lessons, occasionally she'll ask the same random old guy for help like me, and she'll just skate and then promptly leave. One older guy (like over 40, she's 19 or 20 like me) has like, gotten her to practice ice dance with him, but yeah, never brings friends. Almost all girls skating in public have friends that go with them. She just wants to be good at skating, and doesn't care. That's exactly what I do. Also, from talking to her, she seems to be like, thinking, hard. And she said her same problem writing essays is like, too much to write, as I have. Oh, and she said she's taken Adderall before, and drinks coffee and energy drinks constantly, tada, also like me.
You wouldn't guess it, though, that's the thing. It's only after being around her long enough, that I've picked up on the similarities, and that's been months. At first, I just thought she was like, some quiet haughty girl or something (I think the word I may be looking for is aloof), she just came off like that, but then when I saw that she struggled learning skating and was just compensating like I was just by trying super hard, and you know, lots of other similarities. But it took months of me being around her.
But, I think a lot of people make the mistake of nerd=Aspergers, and that's not the case. All these people actually were not "nerdy" at all, and that girl actually dresses and looks really good, same with my friends, both of them are good looking dudes who seem cool upon looking at them, and it's only after you know them a long time that things start coming out of the woodwork that seem different, they might seem somewhat aloof, etc. Me, I never noticed anything wrong with them at all, I just thought "wow they think similar to me, cool!" and just left it at that.