You should make up an excuse for it. You should buy her something like a body wash then show her how to use it by showing her it in the shower or something and then you can see. But, seriously? Go to the appointment! Your daughter is HURTING herself! You should take a footstep!

This is hard, you want to show you trust her but don't want to push her. Maybe have her get a physical exam, normal health reason and inform the doctor of your worries.

Or make her a part of the decision. Depending on your relationship, maybe express your concerns with her and let her know you trust her, but as her mom you will worry no matter what and ask her if she is okay with showing you, just this once, to ease your mind.

Hi, I am so sorry to hear about this issue. You have done the right thing by getting her help rather than hoping it will go away on its own, like some parents do. It is an addictive behavior so acting fast is the best method. I hope the appointment went well and progress will soon be made. However, it is not an overnight cure, the best  thing you can do is be supportive, without overbearing (which will push her away). Also keep a close eye on her, if things get worse or the cutting escalates, put her on a 10-15 minute check and tell her it is for her own safety, not a punishment. Places to check are arms, stomach, legs (especially thighs), places that can be hidden. Cutting is most typically a form of releasing anger or negative feelings, so maybe there is another outlet to find, i.e. journaling, maybe even a punching bag, some way for her to release the negative feelings.
http://www.selfinjury.com may be a great resource for you.
Good luck!