Look for his call. If he is interested, he will want to call to set up another date or just talk. If he doesn't contact you, even by text, he's probably got his mind on something or someone else.
Involve the group. If he takes the time to introduce you to his friends or wants to meet yours, he wants to get the two of you cozy in each other's lives.
Listen to what he says. If he picks up your speech patterns or slang, he likes the way you talk.
Watch how he listens. If he keeps eye contact and leans in while you talk, he wants to make sure he catches it all. Open body posture -- arms and legs uncrossed, torso facing you -- means he's interested. If he remembers things you tell him and brings it up later, he is interested.
Check to see if he treats you like a lady. Acts of chivalry, like opening doors or getting your coat means he respects you.
Take him to do your stuff. A guy who will step outside of his normal behavior to spend time with you prioritizes you. Girly outings, like shoe shopping or watching a chick flick, without complaint means he just wants to spend time with you.
Observe how he talks about you. If he brags or sings your praises when talking about you to others, he's proud of you.
See if he voluntarily does things your way or steps outside of his comfort zone to be with you. A man who bends to accommodate you is interested.