Since I made a nice long compilation of these, I figure I might as well put them somewhere convenient. Here goes.
You know you're a nerd when...
When you write page-long Facebook comments about *serious business*. Heck, when you write page-long Facebook *anything* about serious business.
When your choice of favorite hang-out ends with a TLD.
When Feynman is a household name.
When you verbalize your actions with Net slang. A laugh is just a 'lol', a sigh is just a *sigh*...
When the Rules of the Internet are more familiar to you than the Rules of Fight Club.
When you go to Sporcle to train.
When you prefer Primer to Avatar.
When Neo first shows off his powers irl and you think, "Oh shit, they're embedded in nested Matrices!"
When you are ashamed of your sesquipedalian loquaciousness.
When you decide that referencing xkcd for nerd cred is cheating.
When you bet on the inclusion of one preposition in one line of one book. (It's "Aragorn sped *on* up the hill." Obviously.)
When Twenty Questions really means "Keep asking till you give up."
When the food is problematic.
When you cannot get through the day without your fix of silly cats.
Alternatively, when you disdain silly cats, but think silly dogs are the shit.
When you're good friends with someone you've never met in meatspace.
When you're at the bookstore, and start riffling through a book to see if you want to buy it...and realize, a while later, that it would be pointless to buy a book you've already read, and also that the sun's gone down.
When you consciously verb nouns. Double points if that makes you think of Calvin and Hobbes.
When you make sure your [sarc][/sarc] tags are properly formatted.
When weirdness becomes a goal to attain rather than a quality to shun.
When you are a Timmy, Johnny, or Spike. Melvin or Vorthos? Okay, now you're just a smartass.
When the fate of your debate rests on a fictional planetary ecologist.
When you feel a compulsive need to use xor to remove logical ambiguity.
When you are familiar with the Laws of Poe, Sturgeon, Godwin, Moore, and Finagle.
When TV Tropes ruins your vocabulary.
When you describe Jesus as a subclass of God that implements the Human interface.
When the zeta function is elementary. (Except when it isn't.)
When you think that Transcendentalists must not have liked polynomials very much.
When you worry about the effects of drunkenness on your mental math.
When your response to use of the words pneumonoultramicroscopicsilicovolcanoconiosis, electrophotomicrographically, and floccinaucinihilipilification is something other than "Gesundheit".
When you worry about your use of the Oxford comma.
When your reaction to hearing about Homo floresiensis is to muse that Tolkien was telling the truth all along.
When you join a party by making a charsheet.
When you realize that the Ringworld is unstable.
When you consider the joke "there are 10 types of people in this world, those who understand binary and those who don't," and its higher-base variations, to be old hat.
When your chat with a friend bounces between nontrivial reasoning in philosophical thought experiments and appropriate balance for interacting feats/spells in D&D.
When you start dismissing nerdy pickup lines because the math/science involved is too elementary.
When, in your conversations, the term "lawyer" refers to a gaming pedant rather than the profession.
When the Chinese in Firefly breaks immersion because it's really BAD Chinese, or the dialect changes mid-episode.
When your bedside table reading pile is really a bedside table, three desk shelves, the top of your clothes drawer, and the lower bed bunk--and you leave the room at 3 AM to go find a different book.
When you lose The Game.
When you feel a little sad inside that someone, somewhere, knows more digits of Pi than you.
When hearing a relatively obscure mathematical term causes you to break out in song (Mandelbrot set, anyone? Bonus points if you started singing on 'Julia set' instead).
When you sit down to read all the goddamn codex entries in Dragon Age.
When you start min/maxing your Pokemon's attributes.
When you've been clicking on the 'random article' button for the last two hours.
When you find yourself having to choose between chess and go for the next game.
When you lol irl.
When people compliment you on your Renfaire or cosplay costume.
When you are genuinely outraged that Tom Bombadil got replaced by a random encounter with wood-elves, and then wonder whether those wood-elves were more likely to be Sindarin or Noldorin.
When you have tried to memorize the elements...lyrically.
When you show up to your school's traditional first-day hazing...wearing your self-made chainmaille coif.
When Quenya is your second language.
When you rack your brains thinking of ways to contribute to this list.
(note: about 95% of these are actual self-reference, but not all.)
More will be added below when appropriate.
If, at any time during the last 525,600 minutes, you have mused on the diminishing cost of a gigabyte of memory...you might be a nerd.
Related: The probability that you are a nerd increases asymptotically to 1 with the number of arbitrary unit conversions you have memorized.