If you're talking to a girl on the street, at a party, or anywhere else, it may be hard to tell if she's interested in you. She's probably not going to come right out and tell you she likes you, so you should look for signs of attraction in her body language, the way she speaks, and other more subtle aspects of how she carries herself and interacts with you.
If you catch a woman looking in your direction a few times when you're at a party, bar or other social situation, the odds are she was looking at you, not the things around you. If she goes on to linger with eye contact when she's talking to you, this is just further evidence that she's attracted to you. Of course, this is not an exact science. You may just have something on your face and she's too polite to tell you.
Mimicking body language and voice tone are also signs of attraction. You can see this with men and family members too -- mimicking body language is not necessarily a sign of sexual attraction for everyone, but it is a sign that the person you are talking to wants to relate to you and is interested in what you have to say. So, if you brush your hand through your hair and she does the same, the odds are that she's interested in you.
A woman who is interested in you will often tease you and engage in verbal banter. Most people don't tease their vague acquaintances or strangers, as it takes a certain level of closeness to tease someone without offending him. So, if a woman is engaging in casual banter with you, she probably feels close to you -- or would like to feel closer to you.
You're probably not funny all the time, even if you consistently make people laugh. You probably know when you tell a joke that falls flat. However, if she laughs at all your jokes, even the flat ones, she is probably interested in you. So keep an eye on whether she is the only one laughing at your lame attempt at humour -- if she is, she's probably interested.