Am I a brony, when I know all of the ponies names by heart?
If you can himone of the songs too, you're more than qualified.
Not sure if it counts, since I caught myself singing the animated Titanic movie songs (hey. Is it really my fault if they're so catchy in their badness?)
I'd say it starts when you begin to do some world building in your head, or start discussion about some aspects of the show.
By the way, I have to say that season 4 is off to a good start.