Pay attention to what he wants to know from you - If a guy asks you a lot of questions about your personal life, it shows that he doesn't just want to chat. Then he wants to get to know you on a deeper level. This could indicate that he considers you a close friend, but also that he is interested in more.
Pay particular attention to whether he asks you about private interests, hobbies, friends, family and especially about “feelings”. If he remembers what you told him and comes back to it, it shows that he is listening intently.
Pay attention to whether he gives you a nickname. A group likes to give a friend a nickname when they do something special. However, if someone comes up with a nickname for you on their own, especially if you haven't done anything special to deserve it, then they may want to show how special your relationship is.
Make subtle changes to see if he notices them. If a guy likes you, chances are good that he'll notice changes in your style or if you've accomplished something you've been working on. If he compliments you on this or even just comments on something, then it indicates that he is paying quite a lot of attention to you. For some guys, a simple compliment like, “That’s cool!” means a lot.
Pay attention to whether he is teasing you or trying to make you laugh. Everyone has a male friend who constantly acts like a clown and wants to get all the attention. However, if a guy tailors his jokes more towards you or playfully teases you, then there's a good chance he's flirting with you. If a guy likes you back, he'll probably laugh more at your jokes. Even if they aren't particularly funny.
See how often he wants to spend time with you. If a guy is into you, he wants to spend as much time with you as possible. That's why he might make up excuses to see you. A few requests for meetings per month may not indicate interest, but several per week may be. Pay special attention if he wants to do something just with you alone.