Wil Wheaton, in his Gnomodex presentation, starts with a great opening. In the interview afterwards he gives a few pointers to differentiate between geeks and poser-geeks.
I have rewritten the opening a bit to fit me:
"I am Erwin B. and I'm a geek. I discovered that I was a geek in fifth grade when John Doe (name changed to protect the
imbecile innocent) kicked me in the nuts and I retreated to the relative safety of the bench. Many years later I drove a 300km return trip once a month for a game of AD&D. I've handled my study books, calculator and computer more often than
any athletic implement."
You cannot be a geek if you don't bear the scars, so to speak. If you haven't skipped going after specimen of the preferred sex for a romantic encounter to watch 'Cosmos' or an SF serie instead, you're not either. It also helps was if the main reason for hiding
under the bedsheets so as to not be caught be your parents, was reading a Star Trek photo novel with a flashlight past bedtime. One thing counts against me: once a bully that thought he would get away by spitting at me was sent flying a foot or four by yours
truly (he landed in a stack of backpacks, I'm not very evil, just a bit  ).
If you read the above and think 'hey, that somewhat resembles me', than it's only a matter of the type of geek you are.
My geek is: IT, SF, poker, philosophy, astrophysics.