When they unironically say “based” or use “Marxist” or positively (multiple of my teachers have done this, I’m super lucky to live in a relatively non hell hole part of the US). Also when they say “communist” positively or talk about how they like Cuba (my peers have done these). A more subtle way is if they talk about dialectics or “people’s history.”

Also, the way you’d pick me out is my “toadal commie” shirt, my “death to amerikkka” shirt, my baret with a red star on it, or my Lenin pin on my backpack. That’s by looks, assuming you haven’t heard how I always talk about communism whenever I get the chance.

Edit: almost forgot the pro-union/Sankara/antifa stickers and patches.