What these guys said. If a girl has been dating you for at least 3 months and you haven't given her the "girlfriend" title, if she likes you enough to want to be that with you she'll say the three words all girls say:
"What are we?"
If they ask you this, it translates to "I want to be your girlfriend." But you don't just up and say "Gee, I dunno, wanna be my girlfriend?" You still need to do a few things before you make that decision:
1. Ask her "Why do you ask?" Some girls are asking because they want to be your girlfriend, while girls who want to just be F-buddies want to see if all this dating means you're feeling something deeper. Either way, make her explain herself.
2. Assuming she says something akin to "I want to be your girlfriend" (i.e. "Well, I care about you, and I think you like me, soo...." - they're rarely ever direct and to the point), this is the time to negotiate all the things you want in a relationship. So drill her - ask her if she's cool with certain things that make you the way you are (ex. "Well, do you care about the fact that I can be messy at times? Are you gonna be cool with me hanging out with the fellas at times? Are you still going to cook my favorite meal every once in a while? Is sex still going to be a regular thing?"). You may think it's dumb to do this, but TRUST me - anything you don't bring up now, you won't get later. If you want her to give you oral sex every morning before work, bring it up now - sounds crass, but people have broken up simply for not getting that thing they wanted but didn't ask for.
3. Give her a non-committal answer that lets her know you like her, but that you have no problem walking. I usually answer with "Hmmm... well, okay, let's try it out and see what happens." It's not a full-on "yes," it's a "you're under review to see if this works" answer, one that keeps a woman on her toes as to just how much you like her, while still allowing her to say she has a boyfriend.
Hope this helps!