Be aware of how often he looks at you. Men are very visual. When a man is interested in a woman, he will steal glances at her, watch her when she is talking to other people and follow her with his eyes. If every time you glance at him, you catch him glancing at you, then he is probably interested. Also, when a person is attracted to someone, he tends to blink more often and his pupils may dilate slightly. Watching a man's eyes can be very telling.
Body language speaks volumes. Pay attention to how he stands and moves when you are around. When a man likes a woman, he will send subconscious signals. He may stand a little taller, puff his chest slightly and use his arms to appear larger and to take up more space. These are dominant characteristics, subconsciously attempting to show you how big and strong he is, and that he would be a good mate. Also, he may seem a little nervous around you. He may talk a little faster, make more jokes and laugh more often.
Notice if he seems to be wherever you are. If he finds a way to sit by you, work near you or ask you a question, then he is interested. He may come by your work or a place you hang out often, attempting to see what you are like around others and what you are like in regular life as opposed to a date. If he finds a reason to ask you a question or share something discreetly, he may lean in close to talk to you. This allows him a closer look and the chance to smell you. It may seem a little odd, but smells tell our primitive brain a lot about our world. He will find chances to touch you casually while he is close. Men do not touch women they don't like.
See if he wants to help you. Most men have a primitive need to protect and care for women. Holding the door for you, offering to carry things for you, noticing if your car needs maintenance and offering to fix something for you are ways that he tells you he likes you and wants to take care of you.