I would say that a brony/pegasister would have to be one or more of these-
1. Be a fan of the fan of the show and look forward to the next episode
2. Start wondering about who would make a good pairing (I secretly pair Cheese Sandwich and Pinkie Pie)
3. Start reading fanfics/love stories about the show (The first one I read was a love story with Discord and Fluttershy)
4. Fantasize about what living there would be like
5. Start drawing pictures of your favourite characters
There is probably more that could define a brony but a fan decides if they are a brony or not. I am a pegasister and I am 1,2,3 and 4.