Subtle Geek Detectors

On my way back from the vending machines, I realized I was whistling the theme song to Lupin III (an old anime show) and it occurred to me that I was implementing a geek detector. Had anyone recognized it, they likely would have at least smiled knowingly; possibly they would have started whistling along or remarked, "Hey, Lupin!" But to anyone else, I was just whistling a tune they had never heard.

Another detector I've seen is a polo shirt worn by a co-worker which bears a small CyberDyne logo, just some tech firm unless you know they will be responsible for SkyNet.