Notice how the boy looks at you. If he looks closely, and then looks away - there is clearly mixed feeling. Maybe to you he looks for a fraction of a second longer than the rest - also a sign of warm relations with you.
Pay attention to how he touches you. If he were to take you by the hand or during the conversation, she suddenly falls on your knee, it is a sure sign that he cares about you. But don't forget about macho who used to flirt with all the girls in the possible range.
Observe how he interacts with other girls. A lot of guys trying to flirt with other girls just to get your attention, but it is immediately noticeable when communicating with her, he will occasionally glance at you, expecting some response from your side. Another sign that he likes you, would be to stop flirting after you left. You do, of course, will not see, but you have the faithful friends of spy, who'll tell you everything in minute detail.
He is interested in your hobby. By pure coincidence it turns out that he listens to the same music you love, playing beach volleyball and goes to the Philharmonic. Isn't that a nice surprise? Remember - if he's trying to find common ground, live in it a warm feeling to you.
Best assistant for you to be his best friend. If he's making fun of him when you pass by, throwing ambiguous glances in your direction, squints, and smiles maliciously, he vigorously whispering something in his ear, then all is clear - the guy that called screwed.
He notices slight changes in your appearance: a new blouse, hair done differently, the new lipstick is a clear indicator that he likes you. Also subconsciously, he begins to mirror some of your habits: adopts to your gait, manner of speaking, to bend the fingers.
But sometimes girls are wishful thinking. If you don't take your eyes off him, he can see you for exactly this reason, and not because he likes you. Remember that banter may just be a desire to cheer up a bored group of friends.