I've always thought mentioning things were "20% Cooler" was like the universal Brony Call-sign?
Personally less cartoon-like shirts with MLP villains on it (Sombra, Discord are best choices) are the best to weed out Bronies if you really want to play it like that.
What I personally do is wear my Luna Necklace at all times, not only is it special to me, it also helps me find Bronies. Subtle little things basically can find you other Bronies, you don't have to literally go all out Brony and talk in pony-speak. Just little things here and there.