Check out his body language. Not all guys are good at keeping their nerves under control. If he likes you, he may lean in towards you when he speaks. If his full body, including shoulders and legs, is pointed towards you when he speaks to you, he definitely has interest in what you are saying.
Body movements like fidgeting, taking deep breathes or even looking away can indicate attraction. Physical body changes like sweaty palms or fumbling his words are also good indicators.
Beware of eye contact. If he is looking at you and looks away quickly he may be shy, which could make for an uncomfortable, yet flirtatious conversation. If he holds his gaze on you for longer than a few seconds without saying anything, that is also a sign that he is enamored with you.
Watch for touching. If he hugs you or touches your hand or leg when he is talking to you, those are signs that a guy likes you. Watch how he is around other girls. If he touches them and acts the same way around them that he does you, he may naturally be a flirt.
Watch his friends. Often times when a guy's friends know that he likes you, they may tease him. If you are around, his friends may make it a point to smile when you talk to him or they may even ask you if you like him. When he isn't around, engaging in conversation with his friends may be the time when they reveal that he likes you.
Listen for compliments. If a guy gives you a compliment, that may also be a sign that he is interested in you. Complimenting you on a new hairstyle, an outfit or simply saying, "You look nice today" is indication that he wants to make you feel good.