You saw an 'I love nerds t-shirt and flipped, and then bought it'
Feel free to add more.
EDIT: I'll make a list here:
1. You yell at your parents when they tell you to go play, but you say you're doing homework...during winter break
2. You saw an 'I love nerds t-shirt and flipped, and then bought it'
3. When you get annoyed that you have to explain what half the words you use mean to other people.
4. When you're excited to get a chemistry set for Christmas (I got one last year )
5. When you spend all your time on the scioly forums.
6. When you read xkcd/Geekologie/Penny Arcade for fun.
7. When you can understand most of the inside jokes on xkcd.
8. When you spend most of your spare time reading for fun.
9. When you work ahead in class over winter break (what I'm doing now for Chem Lab, since I also have AP Chem).
10. When you actually know about Google Open DNS.
11. when you can go to a theme park and wonder how many times it would take to ride a roller coster and lose your calculater
12. when you take a calculater to a theme park
13. when you can speak klingon
14. when you are reading this
15. when you are in SO
16. When you make this thread
17. When you get a bird field guide for Christmas.
18. When you're disappointed that it's not a chemestry set.
19. When you are the only one in your class who is aware that intrapersonal communication includes the nervous system, and suddenly the teacher who hates everyone likes you because of it...
20. when you're proud to call yourself a nerd
21. When you make science jokes such as "your epidermis is showing" outside of school and SO.
22. When you ask for someone to pass you the sodium chloride.
23. when you ask your science teacher (who also just so happens to be the cheerleading coach) a million questions on one prediction and annoys her to the point of not teaching us it and just giving us pre-handwritten notes (half of our science class is our science Olympiad team )
24. I know I'm a nerd when I get mad that I got a first, second, third, and third in my most recent competition. Not mentioning 20th in Compute This... dang!
25. When "you" teach your teacher something
26. when u wear the I love nerds t-shirt to school and say thanks to everyone that says ur a freak
27. you r not satisfied with your 4.9 GPA
28. you see you test score "99.9%" and start crying of embarrassment
29. write "tri potassium" as KKK just to annoy you chem teacher
30. When you correct your Honors Chem teacher because he wasn't being specific enough in saying mcΔt instead of mcpΔt. (He got mad at me when I started telling him that he wasn't being specific enough in saying "c" because there is specific heat at constant pressure and specific heat at variable pressure.)
31. When my chem teacher got mad at me because i asked about something we hadn't learned yet
32. When you're in 4 SO events.
33. When you've placed in the top 10 at the state spelling bee . . . and can spell floccinaucinihilipilification, xanthosis, sprachgefuhl, and pfeffernuss without getting a "brain cramp."
34. When you cry when your worst grade is 89.99 . . . missing the math honor society by 1/100 point
35. When the toughest teacher in the school gives you a 'B', you almost cry.
36. When you're the first one done with your homework in computer apps and end up on this site.
37. When you lose the city spelling bee four times in a row (we go straight to nats) to the girl who won last year and get mad that she didn't win sooner... (then she joined our schools' archrivals' SO team.)
38. ur sci oly shirts says embrace ur inner nerd
39. u don't mind being called a nerd
40. u can converse to ur doctor at a professional level and still understand what they're talking about
41. u know more about anatomy than ur bio teacher
42. i had to correct him once about thrombocytes (he forgot the name) (platelets)
43. and the color of blood, and how scars form
44. u freak if u get a low A
45. u must maintain a 5.0 GPA
46. ur lowest grade is a 92
47. When you are solving a crossword puzzle (okay, I could stop with that) and the clue for a 4 letter word is sodium chloride and you think "NaCl" instead of "salt".
48. When all your friends tell you that if you were a real friend, you would miss SO practice and go to their party. But you go to SO practice anyway.
49. When you do 7 events at Invitationals without much trouble
50. you accidentally say "Command: Stop" to a thief
51. When you think of the element lead instead of peanut butter when you see the initials "pb".
52. when you memorize the whole periodic table "for fun".
53. when you become depressed and commit suicide the asian way when you get an A-
54. when youre only supposed to make cards for the first 18 elements and you do it for the whole periodic table of elements because you had "nothing to do
55. When you calculate the fibonacci sequence without a calculator up to Fib100. For fun.
56. You know your a nerd when your friends year book quote is 'embrace your inner nerd' and your is 'reach for the stars'
57. When you read this fourm, if you don't understand one of the jokes, you must immediately look it up.
58. When you love to watch The Big Bang Theory, not just because it's funny, but because it reminds you of your own life.
59. ...when you're more excited about Pi Day than you are about your own birthday.
60. 3.1415926535897932384626433832795028841971...and so on to infinity! (And yes, that was from memory.)
61. I woke up on pi day, and I didn't think, "Heeeeey, it's my dad's birthday, I should get him a gift." No, I thought, "PI DAY SQUEEEEEEEEEE!"
62. When a friend looks at the book you're reading and comments, "WHY THE HECK WOULD YOU READ THAT???"
63. 1. when you have 10 books cheaked out of your school library when max is 3
2. whhen you spend during lunch reading a bookin the media center
3.and when you recite a pert of a scoiloy compatition by memory to your freinds
all i cound think of
64. When you prefer the bookstore to the public library for the sole reason that you know if you check something out from the library, you'll end up racking hundreds of dollars in overdue fees...
65. You know you're a nerd when you are terrible at identifying sarcasm. (Like Sheldon, from The Big Bang Theory! )
66. When you see a license plate with the number 65859 and you immediately think, "Wow, that license plate reads the same way upside-down!"
67. ...when you have arguments with your friends about who has a higher nerd ranking (nerd, supernerd, ultranerd, ubernerd...)
also if you are a trekkie (i'm getting there...). then you absolutely must be an ubernerd at the very least.
68. when you put on the back of your SO shirts...
GAME
69. when you start crying after you didn't get a medal at regionals.
70. when you get chosen last to a team to play basketball or football at lunch recess.
71. when you even glance at World at Warcraft with awe or an "i want that" look.
72. Your science teacher randomly asks you if you are in the chess club.
73. When you tell someone that you do a sport and they say "r u kidding?"
74. When you cry after you got 2 medals at regionals...cuz they weren't gold.
75. When you don't even go outside at lunch recess. ]
76. When you really really wish you could download gunz.
77. When you answer "yes" to above question.
78. When you tell your national mathcounts team that you play soccer and they spend the rest of the practice freaking out. "What?!? [insert name here] plays a sport?!?"
79. When you go out of your way to trump somebody else's nerdiness to prove that you're a bigger nerd.
80. when you cry about getting 4 medals at regionals (2 gold but 2 bronze!!!)
81. When you cry after medaling in all six of your events at state. Yeah, I think I just trumped you all
82. When you want to slit your wrists because your GPA went from a 99 to a 98.
83. When you want to slit your wrists because your GPA went from a 99 to a 98.
Yes that's actually my GPA===When you actually slit your wrists for that
84. When you would rather stay at practice calibrating your trajectory then hang out with our friends
85. When you don't have any friends because you've been calibrating your trajectory!
86. When your friends all ask you on a regular basis, "What's a trajectory?" When you tell them you have to work yours instead of <insert fun thing here.>
87. when your favorite shirt is your team shirt which reads "imbrace your inner inerd"
88. when you get annoyed that the periodic table in your science classroom doesn't have elements 110-112 listed properly (darmstadtium, roentgenium, copericium)
89. when your favorite thing to do right after school is practice Science Olympiad
90. When your girlfriend's name ends in .jpeg
91. when you make up band jokes that are actually funny if you are in band only.
92. When you are on a Cav voices tour (elite choir) and you go on line to check your nerd sites (like wikianswers, wikipedia, and my favorite scioly)
93. when your favorite shirt is your team shirt which reads "imbrace your inner inerd"=====when you have to point out that someone else had a spelling mistake, namely "imbrace" - it starts with an e, earthbot.
)
...NOT Communist.
I still wish I had a chemistry set 
