You can tell a music-theory nerd when:

Your favorite pickup line is, "What's your favorite augmented sixth chord?"
You like to march around your room to the rhythms of Stravinsky's "Le Sacre du Printemps."
You love to quote Walter Piston.
You long for the good old days of movable G-clefs.
You feel the need to end Tchaikovsky's Pathetique Symphony with a picardy third.
You can improvise 16th century counterpoint with no trouble, but you frequently forget how to tie your shoes.
You lament the decline of serialism.
You enjoy the tang of a tritone whenever you can.
You like to deceive your friends and loved ones with deceptive cadences.
You find free counterpoint too liberal.
You wonder what a "Danish Sixth" would sound like.
The "Corelli Clash" gives you goosebumps.
You can hear an enharmonic modulation coming a mile away.
You have ever done a Schenkerian analysis on "Three Blind Mice."
You have ever tried to do a Schenkerian analysis on John Cage's " 4'33" ".
You have hosted a "Gurrelieder" party.
You have ever pondered what an augmented seventh chord would sound like.
Bass motion by ascending thirds or a sequential pattern with roots in ascending fifths immediately strikes you as "belabored."
You know what the ninth overtone of the harmonic series is off the top of your head.
You can name ten of Palestrina's contemporaries.
You have ever heard a wrong note in a performance of a piece by Berio, Stockhausen, or Boulez.
When you're feeling particularly prankish, you transpose Mozart arias to locrian mode.
You keep a notebook of useful diminutions.
Those "parasitic" dissonances make you queasy, especially when left unresolved.
You know the difference between a Courante and a Corrente.
You have ever used the word "fortspinnung" in polite conversation.
You feel cheated by evaded cadences.
Every now and then you like to kick back and play something in hypophrygian mode.
You abbreviate your shopping list using figured bass.
You have ever told a joke that had this punchline: "because it was POLYPHONIC!"
You know dirty acronyms for the order of sharps.
You can not only identify any one of Bach's 371 Harmonized Chorales by ear, but you also know on what page it appears in the Riemenschneider edition and how many suspensions it has in the first seven bars.