Summarize:

If they are constantly staring at you, or in your direction, it may be because they have a crush on you. Try to pay attention to the difference between a normal stare when having a conversation versus a more intense, lustful glare. A lot of eye contact is one way they will try to gain your interest and get you to notice them. When you catch them looking at you, see if they pretend to look somewhere else. This is a good sign they have a crush. Try standing out of their sight to see if they move in a way where they can see you again. For example, if you two are talking in a group of people, stand behind someone so they can't see you. Then, watch to see if they position themself in a new spot that allows them to see you. If they try to stand close to you at parties, or sits next to you at lunch, chances are they have a major crush on you. This is their way of getting as physically close to you as they can to express how they care and enjoys being near you. People express their emotions with their body language, and that includes hands. Girls are known to flip their hair or gently touch the shoulder or hand of the person they are crushing on. Guys tend to talk more with their hands when talking to girls they have a crush on because they are excited. If they blush around you, laugh uncontrollably for almost no reason, can't look you in the eye, or fidgets a lot, then you have your answer. These are all tell-tale signs of someone who is crushing. A common sign of attraction can be shown through imitation. When people do this, they do not even realize it. Watch for someone using common words or catch phrases you say or talking about things you are interested in to relate with you more.
Take note of heavy eye contact. See how close they get to you. Look at their hand gestures. Pay attention to awkward behavior. See if they mirror your behavior.