She'll look at your face with bright eyes and a big smile.
She'll be facing towards you. If you're sitting next to each other, look for her to be tilted slightly in your direction. Some girls don't even realize they do it, but it shows they're paying attention to you.
If you're in a group, she'll be paying attention to what you say more than anyone else. So if everyone is talking over each other and you suddenly open your mouth to say something, she'll be listening and reacting to you.
She'll make excuses to spend more time with you. For example, I went to a movie last week with a group of friends, my crush included. After it ended, we were hanging out in the lobby of the theater, and my friend who was heading out offered to walk me to my car. And I declined because my crush was staying behind and I wanted to hang out with him some more.
I almost always make eye contact when I like a guy. Every magazine I've ever read says to make eye contact with the guy so he knows you're interested. So if a girl keeps looking at you and making eye contact with you, that could be a tell-tale.
If its a guy I don't like then I'll treat him more like a brother: play fights, being crazy competitive, arguing freely with them, talking about the boys I do like, etc.
If its a guy I do like...hmm well I don't ever flirt because I become to shy, but I'll find it very hard to look at them in the face, I'll be fighting a smile, I'll find a reason to leave just so I can take a minute to catch my breath, I'll stumble over my words or act a bit awkward, I'll have no contact but if there is I freeze for a second because of all the butterflies, etc.
I'm not a big toucher. I don't like hugs and I will normally avoid touching everyone. So if I bump into you or brush up next to you or pat you on the shoulder or the head or lean up against you or anything, it's pretty much a guarantee that I'm interested.
I'm always smiling at him and I do things for him to help him out. I'm shy, so it's hard for me to be very open about it. So I don't touch him a lot, but I do know girls get flirty and touchy. I get embarrassed even though I would love to do the same thing. Like the other day I let him know there was a deal on something he is very interested in so I quickly text him to tell him. But I also get shyer and blush a lot around him too. I don't mean for that to happen, it just does but people always tell me that it's a dead give away.