Similar to the gaydar, nerdar describes the ability to detect nerdiness in other people.
Protagonist: Hey, did you know that today is the day of the kiss?
Foil: The sixth of July is the day of the kiss? Six times seven is forty-two, do you think there's a link somewhere to the meaning of the world?
Protagonist: Dude, chill out! You're setting off my nerdar something fierce!