1. Lots of eye contact (or in some cases barely any) and a red face is usually the first sign. Suggesting things in common in conversation and light touching (ex. talking and looks at your arm and lightly lifts your shirt sleeve “Oh is that a tattoo? I have one in the same place!) Most of the time you can just tell, people have a certain look in their eye talking to their crush (compare the face they make talking to someone else vs. talking to you if unsure).
2. Laughing the loudest at your jokes, using your name a lot, always looking at you, comments on every Facebook status, likes on Instagram.
3. After hearing a funny joke and laughing, people tend to look toward the ones they admire/feel close to, to gauge their reaction. It’s not exclusive to crushes but it’s very telling.