When you like someone (romantically or not), you tend to mimick them unconsciously. It's called mirroring :
"Mirroring is the behavior in which one person subconsciously imitates the gestures, speech pattern, or attitude of another. Mirroring often occurs in social situations, particularly in the company of close friends or family."
If you do some researchs about it, you will mostly find articles about how "if your crush mirrors you, he loves you !". Well, while you can be sure that your crush likes you (if he does this unconsciously), you can't be sure that he loves you, he may just really appreciate you as a friend/person. Sorry !
I wanted to tell this fact because it's one of humans' cutest thing, and it can be a way to tell if someone likes you or not !