(This is sort of like how I wonder why I don't get groped on the Tokyo subway. Busty blonde. How is it that I'm immune? And yet, on second thought, not complaining.)
But then I thought about it. The first geek cred check I remember undergoing was when I was 8, and the boys in my class didn't believe that I watched Battlestar Galactica. In point of fact it was my absolute favorite show at the time, and my obsessive, over the top fannishness was as alive and well back then as it is now. I handily proved it (and that I watched the show) by naming not only every major and minor character, but the actor or actress who played them as well. By the time I was running out of names of one-shot guest characters I had earned their grudging respect -- and their sidelong nervous looks too, I suppose.
These days I've gotten so used to being cred checked that I don't really notice any more. If someone is asking me about a fandom I'm into then I look at it as a way of connecting with other fans. If it's not something I'm into, well, there's a whole lot to fandom and I don't expect to personally know every aspect of it. I'm personally passionate enough about my fandoms in particular and Fandom as a tribe in general that I'm not going to let a couple of people saying I'm not a "true fan" because I don't know the Marvel universe (I don't), or can't recognize the latest popular video game character. (Still won't.) If one wants a "true geek" then this, for me, is the measure of it: are you willing to show your passion no matter what, even if people around you may look down on you for it?
Plus, I think it's fun playing "better geek than thou" and winning. Particularly when it's a bunch of guys who initially wrote me off because I have boobs. (They're auxiliary data cores don't you know. Duh. You think I keep all this information in my head?)