Part one: knowledge and skills [computers]
You know how to turn on the computer
You can recall your computer's basic specifications on request
You can use the Internet competently
You know that computers use a thing called binary
You can count in binary
You can upgrade a computer's hardware
You can build a PC from scratch
You can set up a computer network
You can program in BASIC
The only programming language you know is BASIC
You know at least one of these:

Part two: knowledge and skills [science]
You know what an atom is
You know what a [biological] cell is
You know the difference between current and voltage
You understand relativity or quantum physics or both
You could build a radio if you wanted to
You HAVE built a radio
and a transmitter
You understand the theory of evolution well enough to counter illogical criticisms
It's obvious to you how to calculate basic physical values such as speed or distance travelled

Part three: knowledge and skills [math(s)]
You try not to laugh when someone says that they find ratios and percentages hard
You can do calculus
and you know it's easy
You have memorised PI to 9 or more decimal places
You can't multiply or divide by 10 instantly in your head
You CAN do the above - well congratulations, hey so can I!
You can use standard form

Part four: lifestyle
You have attended one or more LAN parties
You have hosted a LAN party
You understand 1337 5P34k
You think leet speak is cool
You have or have had a girlfriend/boyfriend
She/he's a nerd too
You own more than ten games
You remember the good old days of Unreal Tournament and Quake
You regularly play Dungeons and Dragons and/or RPGs
You pay a subscription to a MMORPG
You know of Monty Python
You think the newer Star Wars all suck

Part five: nerd humour
You get the binary joke (there are 10 types of people, those that understand binary and those that don't)
You can make and understand maths and physics jokes
You've seen Spaceballs
You know who Weird Al Yankovic is
You are adept at making puns
You can make dirty jokes from UNIX commands

Part six: others
Since not all nerds are computer or science nerds, here's somewhere for all the linguists, philosophers, politicians, etc to scrore some points on:

Linguists:
You know the names of three language families
You know the name of the main language spoken in almost every country
You speak another language
Number of languages you speak: 1 2 3 4 5 6 7 8 9 10 or more
You know the difference between a language and a dialect
You know what an isolating and an agglutinative language is
You find yourself mentally correcting people's grammar when you read something

Philosophers:
You can name 5 western philosophers
You can explain the difference between rationalism and empiricism
You know how to argue for and against either point of view
You know what 'cogito ergo sum' is and its significance
You think an argument is a fight
You know to use 'begs the question' correctly (look it up before ticking, unless you're certain)

Politics (most of these are from Tom, aka Tomzini):
You know the difference between conservative and liberal
You watch politics programmes sometimes
You know what the correct definition of: democracy, republic, communist, socialist and facist are (and be honest)
You know what discourse analysis is
You can hapily discuss Absolute vs Relative gains
You don't vote and you are of voting age