Women have many tells. Indicators of interest are little things that she will do either consciously or subconsciously to indicate her interest in you. The following is a list of IOI's (indicators of interest).
The conversation stops, and she re-initiates the conversation with you. This can be a good way to test her interest levels, just stop making conversation and check if she picks up the ball.
When girls giggle with each-other, while noticing you, or when a girl giggles when she is talking to you.
If she touches you, it's an IOI.
When she attempts to find out more about you, ask you questions and build rapport.
If she looks at you often, glances back at you every minute or so.
She tosses her hair around to see if you will look at her.
If she (almost as a reflex) touches her hair when she notices you. This is a subdued version of the hair toss.
She looks at you and smiles.
If she stands close to you, not facing you.
She joins in on your conversation or laughs at something you said from nearby.
If she brushes up against you while walking past you.
She opens you. Asks if you have a lighter, or finds any way to initiate a conversation with you.
If you're talking to her group, she is especially talkative (in an effort to get your attention).
She asks "Whats your name".
She asks "What's your age".
If she gives you any kind of compliment.
If she is flirtatious, playful or tries to challenge you.
If she is hating (disagreeing) what you're saying but laughing at it at the same time.
If she punches you in the arm while laughing.
She invents a nickname for you.
While talking to you, you notice that she is playing with her hair a lot.
When she is sitting next to you, her leg may touch yours and she leaves it there.
She inquires if you have a girlfriend or not.
Without knowing if you have a girlfriend or not, she mentions your girlfriend (testing to see if you have one).
She leaves and comes back. (IE, leaves to go to the bathroom, but comes back).
If she holds eye contact for long periods of time while she talks to you.
If she has a boyfriend, she avoids mentioning him.
If it comes up that you like something, she will mention that she also likes that thing and needs help being shown how to do it.
She looks to see what your reaction is whenever she does or says something.
She looks at you from the corner of her eye (to hide that she is looking at you).
When she runs into her friends, she introduces them to you.
She buys you a drink.
She teases you, calling you a heart-breaker or a player.
When she leaves a venue, she approaches you to tell her that she is leaving (number close her).
If she notices you leavings, she asks where you are going (invite her).
When you call her, she calls you back.
She makes up reasons to spend time with you.
When her friends go to the bathroom, or to the bar or to get some drinks, she stays.
If you move, she also moves with you, or she waits for you to come back.
When you get close to her physically, she doesn't move or flinch or pull back.
When you do make a move on her, she doesn't resist, or pretends to resist by giving you some token resistance.