Listen for any questions she may ask about whether or not you have a girlfriend. If she likes you, she'll want to find out if you are available. If she's not interested, though, she probably won't. She may not necessarily ask you directly, but she may try to "tease out" whether you have a girlfriend or not by mentioning her.
Look to see if she's often around. If you find yourself constantly running into her, this is probably not a coincidence. If she likes you, she'll orchestrate things so that she's in the same place as you more often than not.
Look for physical contact. This is a tough one because some girls are just naturally very touchy-feely, so look to see if she touches you more than she touches other people (especially other guys). If you notice her often touching your arm, leg or any other body part, though, then she probably likes you.
Listen to hear if she's laughing at all your jokes -- even the bad ones. No matter how funny you are, there is no way you are funny all the time. But if she laughs at every single joke you make, she probably likes you.
Notice whether she's asking you questions. If she just wants to be friends, she won't be overly interested in finding out about your life, your job, your family or other things. However, if she is into you, she will ask you a lot of questions about yourself. Again, you should compare -- ask yourself whether she asks you as many questions as other women who you know aren't interested do, such as your female friends, waitresses and other women you aren't going to date.