I think the best way to is just to bring it up casually. Say something like, "My skin has really been bothering me lately" and see how she responds. Most parents want their kids to be happy and understand how it feels to have skin problems.
Tell her that you've been researching it and acne isn't just something you have to "grow out of" and that there are a lot of medicines that a dermatologist can perscribe that will help.
A dermatologist is not going to prescribe accutane unless you exhaust all other possibilities. So do not expect to go on accutane the second you get a derm. Your cure might not have to be something so drastic, which is a GOOD thing.