This is a great question and one that I have personally struggled with on several occasions. As a Plastic Surgeon, I can usually tell when someone has had something done. I have also had doubts about whether or not someone had something done.
Personally, my response usually depends on the relationship with the other person and the setting. But, generally speaking most women seem to be happy to hear "You look great!" from a Plastic Surgeon. Stay away from "something about you is different..." or "Did you change your hair style?"
As a Plastic Surgeon, I find that most of my patients are proud of their Breast Augmentation and are proud when someone notices. Most of my patients are not shy about sharing their story with other women. They usually are fine talking to male hairdressers, but other than that they don't discuss their surgery with men.
So my advice, TREAD LIGHTLY! Compliments are always appreciated (by the patient and the surgeon), but don't go overboard with your praise. A simple "You look great!" and then follow her lead will usually be appropriate.
Keep in mind that taking social etiquette advice from a nerdy plastic surgeon is like taking financial advice from your Uncle - made a lot of sense when you heard it, but then when you put it into practice - OUCH.More questions on manners and etiquette: