A list of common behaviours that indicate a girl is flirting with you. If you know one that isn't listed, add it!
If a girl likes you, she'll go really quiet when you approach her when she has her friends with her. She might feel awkward talking to her friends with you listening because she likes you.
Try and catch her staring and give her a smile. If you do catch her, note her reaction. Does she look away immediately? That's a good sign. Does she smile and/or blush? Another good sign.
If she volunteers to sit and watch you performing a tedious, repetitive task, she's probably into you.
Touching! If the girl is touching you while she's laughing at your jokes, or listening intently to your story, it's because she's interested.
Flirting: making serious eye contact, body is pointed towards you, making a real effort to have a conversation and get to know you better, maybe throwing in a compliment or two too. And if a woman touches you, if it's either a hand on your arm or knees touching under the table, you're in.
Touching!- Headbutts, Hugs, Soft Punches, Adjusting clothing or hair, piggy backs, arm wrestles grabbing and pulling are all sure fire signs you're in. On the other hand the friend zone is that 5 foot no touching radius that she is maintaining.
Ask her how she is when you go up to her. If she likes you, she will attempt to keep you talking longer. If she seems happy talking to you even when you're discussing something like the weather, her head is definitely in cloud nine!
Pretend she is a man (assuming you are straight). If your interpretation of the interaction is unchanged, she's being nice. If all of a sudden you feel uncomfortable, she is flirting. Point being, you probably already know whether or not she is flirting - people just have an unfortunate tendency to see what they want to see, and not what is actually happening.
Flirt back. If she responds well then regardless of whether or not she was flirting before, you're set. If she responds poorly then stop flirting.
You can figure out if someone is into you by noticing that the person is overly affected by your presence - people who are interested in you tend to consider what you are doing a great deal more than someone who isn't interested - and that comes out in a lot of different ways.
Look at the position of her feet. If a girl comes and sits down, look at the position of her feet; this could give away what she is focusing on. If her feet are pointed directly at you, that's a good sign! Don't worry too much if her feet are pointing in another direction
My way of telling if a woman is flirting with me is to give her an easy out if she wants to move on and if she doesn't take the out then she is interested.
PROMPTING questions, not just answering them.
Notice whether she is shy, flustered, or giggling. If she seems shy, is slurring her speech, or is saying "umm" or "hmm" a lot, it's probably a sign that she's nervous. If she needs to think about simple sentences longer than it normally takes her, that also tells you she's nervous. Nervous mostly likely means crush.
Pay attention to her speech. If she is talking slowly, or abnormally fast, it's a sign that she is pretty nervous. This is good news for you, because you may be the source of her nervousness.
A big hint is when you ask her to hang out and she makes an excuse. A girl who really wants to spend time with you will suggest an alternative time!
Does she fidget? When she's around you, does she start fidgeting with things around her, like fixing clothes, or playing with her cell phone, or wringing her hands?
In conversation, she will either focus on you, or if she is shy around you (which is good), she will try to have a third person to make it seem less conspicuous. In this case, though, she will still focus on you, and return to making you the center of conversation.
If a girl says "awwww" after you do something, it's usually a sign that she is trying to flirt with you. She does this because she thinks you're cute and wants you to know it. She may do this after you cough, trip, or say something cute.
The only 100% way of knowing is comparing her behavior to you with her behavior towards other males. If she is more touchy-feely with you, laughs more at your jokes than others', and tries to arrange things that are just the two of you, odds are she's flirting.
See how she interacts around other guys. Versus you.
So, if you're interested in someone but they're not being forward about it, you have to gather glimpses of their behaviour and consider how much of it is affected by you being there.