Just simple. Just say the stuff you know a brony has. You know you're a Brony when you have several Pony themes in your Computer.
If you've amassed over $1,000 worth of pony merch, you just might be a brony. Or a raving lunatic, like yours truly.
...if "love and tolerance" has become your life motto and you spend time trying to think deeply of the 'elements of harmony'.
If you're playing video games and imagine all the characters ponified, you might just need help...I mean, be a brony.
If you print out a pic like this and stick it on your wall as motivation to get through the day, you might be a brony.
You know if you're a brony when instead of singing Happy Birthday to you... You start belting out "Make a wish it's your birthday!" And start dancing like Pinkie and Cheese!
If you say "IT'S MORPHIN' TIME!!!" at a brony convention, you're either a brony or a Power Rangers fan. Or in my case, both.