Common signs

Well what I do is I always look at the guy or try to be next to him. Or if a girl is constantly trying to talk even if its small and smiling.

If she touches your arm or will reach for your hand. Spending more time sitting next to you, turning her whole body if possible to face you (this shows she's interested, but doesn't show you whether she's interested in you as a date or just in the conversation) if she stares at you, or you catch her looking at you.

oh! the look down. you'll be talking to her, and she'll do this cute thing where she puts her head down and smiles, then looks up and gives you the eyes. hard to explain, but every girl does it. it's like the cutest thing you can do with your head as a girl.

Facing or leaning towards you is one sign. Smiling or grinning while she's listening to you. Keeping eye contact and not looking away or looking around for something else to do. Laughing at your jokes, especially any dumb ones :) Running her hands through her hair or licking her lips (girls know this is sexy so they'll do it if they are flirting).

With women it is different then it is for men. body language is very difficult to read sometimes and the location you are at will make a difference. Touching- The longer the contact you have with the person the better. lower arm and wrist is good but shoulder and upper arm is more saying friends. any contract with the face is always good. chin and nose. The back- the Lower back is good. the upper back is just saying friendship. If she leans her head on your shoulder that means she feels safe with you and likes you. Younger girls might do light hits to you or try jump on you. That is always good.

Touch. I've always found that a woman that makes any sort of excuse to touch you, usually is interested in you. Obviously this doesn't apply if you know the girl is extremely into hugging people every time she sees her friends or something, but it does apply if at school or work or something and she finds a reason to grace her hand against your arm etc.

It definitely depends on place, environment, people around but regardless of all those, there will be clear signs. It could be as subtle as a twinkle in her eye, or simply the way she is looking at you, or it could be the opposite where she would shut down because of being nervous around you. Just watch her and you surely will notice.