Not only do guys like to flirt generally speaking, but they are always assuming that girls are flirting with them, much more often than they actually are. Furthermore, a study at the University of Kansas found that girls are much less likely than guys to be able to tell when the opposite sex is flirting with them.  Much of this is most likely due to the hesitance on the part of the girls to accept the sometimes subtle flirtatious hints of the guys. There are more direct ways of flirting and showing interest in a person, and then there are those little things a girl needs to know to pick up on as well.
Some of the more subtle ways a guy flirts with a girl include a raised eyebrow, positioning his body to face yours and fidgeting with his clothes, hair or adjusting his pose. Certain signs that a guy is flirting are involuntary, meaning it's something you notice that is happening but that he isn't doing intentionally. For example, a guy might noticeably blush,  and you can also tell if after having a conversation with him, he remembers things that you tell him. Also when it comes to conversations, he's flirting if he starts trying to identify with your interests and find common ground. 
There are also more obvious signs that a guy is flirting with you, like prolonged eye contact and any form of bodily contact. Some guys are bigger flirts than others, so when it's hard to tell when a guy is flirting, look for those little things that stand out to you. Otherwise, trust your more ambitious judgment, as it appears that girls tend to think guys aren't flirting near as much as they truly are.