When you say you can't prove she is drunk, what do you mean? Do you know for sure she has been drinking, but you can't prove if she has drank to intoxication? Do you/did you spell alcohol on her breath?
Here are my thoughts...
If you know for sure she had alcohol, I don't care how much she had, if she drove, she needs to lose that car of hers. If it's in her name, you have a right to take it from her until she's 18. If it's in your name, you can keep it from her until she shows that she is done with her alcohol experientation.
If you didn't smell alcohol on her, yet you could tell she was under the influence of something, you may want to find out of she's using some other kind of drug.
Try not to be too harsh, especially if this is the first time she's done it. The last thing you should be is the enemy. Obviously, you have to make a serious statement to her about how wrong it is to do what she is doing. But yelling and screaming NEVER stopped a child from engaging in this kind of behavior. I would wait until she sobers up to talk. Sit down with her and try to be civil...if you don't talk calmly, she will clam up. Ask her why she didn't call you to come get her. Tell her you would rather her call you than to drive drunk. That takes you both a step beyond the "were you drinking" question to which she can answer "no, of course not." Asking her why she didn't call you to come pick her up tells her that you know she was under the infludence, and that you are trying to get past the offense and get to safety issues - which I'm sure is at the heart of your concern.
If in your conversations with your daughter it comes out that she is doing this a lot, then I would suggest that you get her some help while you still can. If it's a minor problem still, meaning if she's only done this a couple of times, a good therapist may help her and you get to the bottom of this. If this is happening a lot, then she is still young enough and under your control - you can force her into rehabilitation and/or a 12 step meeting. If you're lucky and this is the first time, then perhaps a heart to heart talk will stop this. If she's only done it once, then it's not all that shocking. Right or wrong, most kids have tried alcohol before they graduate highschool. For most, it's a one or two time thing...simple curiosity. Odds are, that is the case with your daughter.
The most important reason for NOT yelling and screaming at her is because you need to convince her that if she were to go out and **** up again, that she can CALL YOU to come get her. She needs to know that if she drinks, she has the option of calling her mom or dad and that she won't get beat up, yelled at or lectured. Promise her that if she calls you under such circumstances, you will pick her up calmly, and you will wait until the next day to discuss the issue. This is SO important because no matter how mad you are, you want her to stay alive and you don't want her to get behind a wheel...hers or her friends...in an intoxicated state.
I sure wish you the best. My daughter is 12, and I'm dreading like crazy going through all this. Please don't judge your daughter harshly. Peer pressure is overwhelming for some kids. Getting drunk doesn't mean she's bad, and it doesn't mean you're losing her. If it was a one time mistake, take this chance to talk with her. Be careful not to lecture or pass judgement. She has to know you love her, that you understand why she did it, but that it was wrong, and you have to take action and punish her. The most important thing is that she has to know she can come to you if she is out with her friends and needs a safe trip home. If you earn her trust by being somewhat cool and calm about this, she won't take it as a free ticket to do what she wants - she'll take it that you are listening to her and trying to help her make good decisions without hating her or judging her. Of course, you don't hate her, but right now, she is tossing and turing in her sleep thinking that you do. I'm a firm believer that dealing with a crisis like this in a calm way will work in the parents favor...and the child's. She's not expecting you to be calm...you will FLOOR her if you are nice, and you may find that you have the best talk you've ever had with her.
Hang in there and the best of luck to you both!
I think I would have been thrown out of the house!!!! I KNOW I would have been grounded until I moved out of the house. My life would have been hell if I pulled something like that. No prom. No Grad Nite. No senior portraits. No boyfriend. No graduation party. 
