I've long held an idea about nerds (geeks, whatever...) that one way you can sort between the two is by which classic sci-fi series they cling to, Star Trek or Star Wars. The key difference lies in the story telling. Star Trek is all about the details. They want to know different technical and social details, how does the graviational system work on the Enterprise, what is the cause and effect of the social interactions, how does the teleportation work? Star Wars is all about the story. Who cares why there is gravity in the X-Wings? What matters is that a Jedi can destroy a ship the size of a small planet with one. How does a light saber work? By jabbing it into the chest of your enemy, that's how.
Being on the Star Wars side of the fence, I may be a little biased in my assessment, but the way this difference plays out in real life is priorities. Star Wars nerds are interested in what is important and fun. Star Trek nerds are interested in the things that nobody in their right might would ever find interesting.
I have found that when I meet a fellow nerd I can determine which of the two series they prefer based off of the previous paragraph. There are so some subtle cultural difference that I don't know how to properly articulate, but can be picked up on in time if you pay attention.