Smiling and Looking
Men seem like complex creatures, but more often than not, they try to be very clear about their intentions. If a guy is looking at you and smiling, he likes the look of you and would probably enjoy saying hello. Smile and look back at him, this will encourage him to come over and talk to you, if you wish.
Getting to Know You
When in conversation with a man, if he is asking you questions, he is into you. He is interested in what you have to say and wants to learn more. If the conversation is more like you are asking questions and he is giving short answers and asking no questions in return, he is probably just trying to be nice but isn't interested. A guy that is interested in you will want to try to figure you out.
Guys who are into a woman will often look for excuses to get her number, and will either ask for it or try to hint so you will offer your number. They may also touch your arm in a nice, friendly way. Touching back in response will encourage him further.
If you are curious if a guy likes you, most of them appreciate being asked directly. Most guys are open and honest about how they are feeling at the moment and will say. They also don't want to scare you off just in case you might be uninterested. Give a guy positive comments if you really are interested.
If you are talking with a guy you do not know very well, refrain from giving too much personal information right away. If someone is coming on very strongly right off the bat, it might be interest in something else besides your personality and wanting you to care about him. Be cautious but have fun.