Note the length time that she spends with you.
A girl who is interested in a guy will show that she's interested in him by spending time with him. If this girl flirts with you for a short time then moves on to the next guy in the room, it's likely she is flirting for fun. But if she spends more time flirting with you then any other guy, she likes you.
Analyze the conversation that you shared.
An innocent flirt will talk to just about every guy in the room making small talk about casual subjects such as the weather. If she likes you, she will be interested in talking about you. She might ask about your hobbies, your work or school subjects. This is her way of showing you that she wants to get to know you better.
Note the eye contact.
An innocent flirt will make eye contact, but it will be brief. If she likes you, then she will stare into your eyes meaningfully.
Note the tone of voice she uses.
An innocent flirt will maintain a normal tone of voice as she's flirting. If she likes you, she will lower the tone of her voice at times. This is so that you have to lean in to hear what she is saying.
Flirt with her.
Let her catch you staring at her lips after you gaze into her eyes. If she mirrors your behavior, chances her she is wondering what kissing you would be like. You might catch her running her fingertip across her lips, or using a straw to sip her drink.
Watch her body movements.
If she's interested in you she will subconsciously mirror the way you sit or hold your drink. If she's just flirting for the fun of it, her body movements will be different than yours.