It doesn't have to be anything very obvious--maybe you start saying "y'all" instead of "you guys"--and notice if it starts popping up as part of his vocabulary. This is a way of subconsciously signaling you are on the same wavelength and try to get you to like him. If you have a slight accent, he may begin mirroring your accent without realizing it. Does she always keep things very light, with a lot of joking and talking about movies or a difficult assignment--subjects she'd probably discuss with anyone? Or has she begun to confide in you a little bit, telling you more personal things or getting really deep into her beliefs or hopes for her future? If so, she wants you to take her seriously and to build a deeper connection with you.  If she looks upset or you know she's had a bad day, make a point to let her know you're available to talk, and see if she confides in you. If conversations are somewhat surface, but she gets over-the-top excited when you have things in common, she's still trying to connect with you. So when your chat about movies reveals you both love Jason Statham and she is overjoyed, she's showing that she's a good match. Ask your friends if your name has come up in conversation and see if they can find out from his friends if he talks about you a lot. If he likes you, he's going to be curious about you and what makes you tick. So if he randomly asks your friend who is on her way to basketball practice "Oh, isn't [your name] on the team, too?", he's very interested in you.  If it turns out your name often comes up in conversation--maybe relaying something funny or interesting you said--it's a sign that you're on his mind. He may even tell mutual friends that he's really interested in getting to know you, hoping they'll tell you.
Start using a word a lot and see if he begins saying it, too. Evaluate the depth of your conversations. Find out if he's been asking or talking about you.