Pay attention to the eyes. Eye contact is one of the major signs a shy guy likes you. Basically, if he's interested in you, he's going to try to do everything he can to look at you. However, be aware that once eye contact is established, he may look away for a second out of a feeling of being overwhelmed or intimidated at the situation.
Look at the cheeks. Blushing is another one of many signs a shy guy likes you. So, when you get a chance to talk to him take a look at his cheeks and forehead. If they are red, chances are he's interested in something more.
Check out his body language. Smiling and laughing are additional signs a shy guy likes you. He may also cower at physical contact... at least until he gets to know you better.
Analyze how he treats you. Shy guys tend to be very polite. This means you can expect him to hold doors open for you, or buy you things, (when the relationship gets more personal).