When no one watches movies that are based on popular book series (Harry Potter) with you because you yell "That wasn't in the book!!" at the T.V.
When the only time you were ever popular in Middle School and High School was when they played Trivial Pursuit as review. People would argue whose team you were going to be on.
When you are banned from watching LotR because you are constantly repeating the lines.
When your SW-fan son bans you from watching SW III-Revenge of the Sith because you make fun of Anikin the whole time.
Have you ever heard of Darths and Droids, by any chance? Google it if not, I think you may find it amusing.
...when you read Irregular Webcomic for the footnotes about quantum physics as much as for the storyline.
...when you study chemistry in your free time.
...when the in-jokes between you and your DH include "Why a spoon? It's DULL you TWIT, it'll hurt more!" and "Noooooo Potato-Domo!"
...when your Animal Crossing house is not complete until you have the phone booth, just in case it's really a TARDIS.