There are many varieties of "geek", and not all are visible. I think that the primary reqirement is the desire/urge to learn and explore, and a relentless curiosity that can be turned inward or outward.

Some geeks have social skills, some geeks are happy to take breaks from computers--and some don't even like computers. Caffeine is (shudder) optional..

A geek knows a geek when a geek sees a geek because there's frequently one of two looks in the eyes of a geek. Either inroverted thought where nothing in the room matters, or extroverted searching- "What's here. How can I use it?" Frequently geeks are scatterbrained about appearance, lending additional clues- socks that don't match (they're both white. Who will notice?) clothes that don't fit quite right (who has time to find pants of the perfect length? An inch or two off is fine.) But that doesn't always apply, either.

There's no single sure sign of a geek, but there are a lot of signs that, when seen, can pinpoint someone immediately. Unfortunately this variety of signs also leads to finger-pointing. "You're not a geek!" and "You're not enough of a geek!" and so on by the so-called elite. Bah.

Should a good geek be able to mingle with normal people and not be noticed? Can one pretend to have good social skills (virtual social skills) in order to fool most of the people all of the time?

Geekish behavior isn't limited to just computers. Could be anything one is passionate or is highly interested in learning anything about. The knowledge or skill gained from those expereiences is something they know or can perform well.

I'd like to think I have some social skills. :)

I like caffeine, but I don't think that's a requirement for "geekhood" (not to say there aren't coffee geeks).

If a geek walks into a room, and no one notices ...

Geeks read what they will and listen to what they will as well. I think the difference between a person who just enjoys reading or listening to music is that the geek might tell you what authors influenced the writer of the book you're reading, or what musicians influenced the artist you might be listening to. In quite a bit of detail. And the geek might even have some kind of trivia to go along with those facts for fun. :D

You are in this tribe?

I think the best "field guide to geekery" is "A Portrait Of J Random Hacker" from the MIT Jargon File:

info.astrian.net/jargon/A_...om_Hacker/

You can't miss them! Its the clothes and just listen for a moment, and you can pick one out of a crowd in a room, and then there is the hair, and if they wear glasses, you know thier going be out dated! You can't miss that. Then just throw out a gaming Q, and you have them... like " What does it mean you add 2 the throwing skill in dex? " You will instantly get asked about what system, and character etc.. or you can ask if the warp theory being researched by Stephen Hawkins has been proven yet.. that gets them started to.. :)
SO! You don't need a book to pick them out..:)