I have QUITE a few:
You know you're a brony when..
1. You suddenly like Dubstep
2. The Brony fandom increased(because of you spamming your friends).
3. When you lift something heavy, you say: "Watch out, here comes Tom!".
4. You care more about Princess Cadence and Shining Armors wedding more than Kate and William's.
5. You remember Canterlot, rather than Camelot
6. You react to bad things by saying "This is the WORST. POSSIBLE. THING!", and then proceed to find a couch to faint on.