1. Fascination with form-vs.-content jokes, paradoxes, and humor having to do with confusion of metalevels (see meta). One way to make a hacker laugh: hold a red index card in front of him/her with "GREEN" written on it, or vice-versa (note, however, that this is funny only the first time).
2. Elaborate deadpan parodies of large intellectual constructs, such as specifications (see write-only memory), standards documents, language descriptions (see INTERCAL), and even entire scientific theories (see quantum bogodynamics, computron).
3. Jokes that involve screwily precise reasoning from bizarre, ludicrous, or just grossly counter-intuitive premises.
4. Fascination with puns and wordplay.
5. A fondness for apparently mindless humor with subversive currents of intelligence in it -- for example, old Warner Brothers and Rocky & Bullwinkle cartoons, the Marx brothers, the early B-52s, and Monty Python's Flying Circus. Humor that combines this trait with elements of high camp and slapstick is especially favored.
6. References to the symbol-object antinomies and associated ideas in Zen Buddhism and (less often) Taoism. See has the X nature, Discordianism, zen, ha ha only serious, AI koans.
See also filk, retrocomputing, and A Portrait of J. Random Hacker in Appendix B. If you have an itchy feeling that all 6 of these traits are really aspects of one thing that is incredibly difficult to talk about exactly, you are (a) correct and (b) responding like a hacker. These traits are also recognizable (though in a less marked form) throughout science-fiction fandom.