Notes

He looks at you a lot. His eyebrow may even lift as he watches you ("the eyebrow flash" that lasts a fifth of a second). He might not even be really conscious he is watching you as much as he is.
Notice how much he looks at your face and makes eye contact.
He leans towards you a lot. Personal space invasion is a sign of major interest.
Check the direction of his hands, feet, legs, toes, etc. If they're pointed toward you, it's a subconscious indicator of his interest in you.
He starts grooming himself. He pulls at his tie to straighten it or he readjusts the fit of his sweater. Perhaps he runs his hands through his hair in attempt to tidy it or he reaches down to tie his shoelaces. Doing this repeatedly is similar to the actions of a male bird preening up his feathers for a display!
Check out how he's sitting. If you see rather manly gestures, such as sitting with open legs or placing his hands on his hips, he's trying to impress.
If he likes you, you may see that he rarely turns his back to you, often leans towards you, and also looks at you a lot. If he slouches his shoulders when near you, he's romantic and cares about what you have to say. If he points his shoulders and pelvis towards you while sitting, he is definitely feeling something for you.
A gaze that has him looking left, then sweeping over your face, then looking right is a sign he's very attracted to you.[2]
Be careful not to confuse a shy guy's darting retreat from eye contact with a guy who is clearly disinterested. A shy guy who is interested will continue to steal glances at you. Be patient!
If you don't like the guy, it can be uncomfortable to maintain eye contact; break it off quickly and scan the room yourself, as if looking for someone else.
When he's around you and he says or does something funny and everyone around laughs, his eyes will flicker towards you for a second to see if you laughed too––this means he's keen to make a good impression on you.
His pupils may dilate if he likes you, but this is quite hard to pick up on, and you might come across as acting strangely by looking that closely into his eyes. If you're around him for a long time, it could be easier to pick up on gradually.
Gauge his interest in what you have to say. It really doesn't matter what you say, it's how you say it that can tell you a lot about his level of interest. So, try this: Lean in and whisper, with your shoulder barely touching his and say something softly. To heighten the impact, steady yourself gently by brushing your arm across his back. If he moves his head closer toward you and either touches you back or maintains eye contact, he's interested in you. If he isn't interested, he'll probably step back or be very unresponsive. A really disinterested guy might even try to shoo you out of his personal space!
Be aware that some guys flirt with other girls to get your attention. It gives him a chance to see your reaction, and helps him know if you really do like him or not. (Yes, it's an odd sort of logic, especially since you might end up so offended or confused that you just give up on him!) However, you can usually spot a "get-your-attention" flirt if, in the middle of his flirting scenario, he keeps taking the chance to look at you, seeking out your response. You can also try a quick trip to the bathroom and find yourself a sneaky observation point to check out how the flirting is proceeding. If he stops the moment you've left, it's you he's serious about, not her. Alternatively, ask a friend to do some observing for you while you're away.
Watch for him showing a sudden, previously unexpressed interest in things that you like and do. For example, if you like a certain genre of music that he doesn't know as well, he may ask you to suggest bands or artists for him to listen to.
If they know he's interested in you, they might tease him subtly when you're around, hint to you that he likes you, or even try to find out if you like him.
Pay attention to gentle, friendly teasing. If a guy teases you in a friendly and fun manner, it could indicate his interest, especially if he's young. Provided this isn't his modus operandi with every girl he flirts with, it's a sign that he's singled you out for attention and is trying to use his wit to charm you, in that awkward way of using humor to cover up true intentions. Of course, if he says something unkind or unfortunate, don't feel you have to excuse it––point it out if he offends you; it's best he knows now that you won't take nonsense than to discover it later.
Acknowledge any compliments that come your way. If you do your hair or makeup differently one day, and the guy notices, that's a very good sign that he likes you. Most guys won't notice, or if they do, they won't be bothered to say anything unless they're totally into you. Anything like, "You look nice today," "I like that shirt," or even "Did you do your hair differently today? It looks nice," are all indicators that he could be interested in you.
If he chats to you often on a social networking site, it may mean that he likes you. After all, when online there are many things competing for his time, like games, connecting with mates and checking out sites that might not be so interesting to you… Of course, he could also just be talkative, friendly, or even bored, so don't assume he's into you just for chatting––this should be treated as one extra indicator along with some of the others outlined above.
If he puts an 'x' sign on the end of everything he says to you, don't assume it means he likes you. It could just mean that he perceives you both as good friends or that he's just used to ending his chats that way.
If he says, "I know something you don't know," then this could mean that he likes you and is playing a little guessing game. Or it could just mean he's doing the usual social media thing of spreading information for fun, like the discovery of a new planet or finding the cure to cancer, etc. Use the context to judge the import of any such online personal game playing.