When your phrase of choice when your BF isn't listening is '...then the Borg came and we were all assimilated'
When you tell non-Star Trek fans jokes like:
Why did the Borg cross the road?
It had assimilated the chicken!
And laugh uproariously... and when no one gets it, you explain in great detail. And the next time you tell a Star Trek joke they laugh just to stop you launching into a half hour explanation of why it's funny!
...you understand most of the science jokes on The Big Bang Theory and totally understand Sheldon's reaction to Penny's Christmas napkin.
I LOVE that! One of the best episodes! "I have the DNA of Leonard Nimoy?!! I could grow my own Leonard Nimoy?!!"
... and that's another one: when someone mentions a geeky show you love, and you can trot out lines from it without thinking twice.
... when you can tell the Star Trek/Stargate/insert-sci-fi-show-here episode from the first 15 seconds.