a shy guy and a one who is just not interested.
get to know him. this is perhaps not the easiest way to know whether he is not interested or just shy but it will lead you to a more accurate conclusion. if he has an established reputation among his friends and family as a naturally shy person, then it will be easy to tell if he is interested with you or not.
body language. you need to observe him closely whenever he is talking to you. a shy guy would be fidgeting or he may not be able to look you in the eye but a guy who is just not 