There any many reasons you can tell, but in each point I will make, the polar opposite applies as well as guys are different.
1. I make more of an effort to keep eye contact. However, some guys could avoid it so loads of eye contact or none at all could be sign (although the latter could be boredom!)
2. I tend to disagree with a lot she is saying, try to get a discussion going, without being rude, so when I leave she will think about me and the conversation. However, some guys could agree with anything you say as love you, so either extremes and you have a chance.
3. I also have the habit of moving away if she touches me or if she brushes against me jumping away. It's really weird. I long for more touches but I can't help it. The other extreme is that the guy could just want to be hugged all the time or always hug you, touch your hand. So either one and there is a chance.
4. If I having a conversation alone with her and somebody interrupts that will visibly annoy me, as i'm enjoying it and trying to get in there. So if somebody joins your conversation and he bottles up that could be a sign, but its a longshot.
5. There's there standard procedure of you catching the guy looking at you. I actually deliberatly try to get her to catch me looking and then hold the look before looking away and trying to cooly rejoin a conversation around me. Equally if he stops what he doing to talk to you or smile at you that's another sign.
That's the best I can do.