Half or more of the many massage parlors are fronts for illegitimate services. As much as I would like a real massage, I haven't gotten one because I don't know how to say, "Real massage only" in Chinese. And I would feel rather silly saying it in front of a bunch of giggling girls. The "extra service" girls probably give a lousy real massage anyway.
Prostitution is widespread but nonetheless illegal in China. There are enforcement actions from time to time. I have had no police contact that involves anything more serious than bicycle parking, and I intend to keep it that way. There is no presumption of innocence in China.
Questionable massage parlors shouldn't surprise anyone, but the same is true of hair salons and barbershops. If there is never anyone getting their hair cut, that might be a clue. What kind of a haircut might an extra-service girl give you? And: you don't know where those hands have been!
My advice: Get you a nice girl, and keep her if you can.
