Yeah never ask, for all they know you could be anybody trying to catch them out, close them down or arrest them.
A lot of the time you either need to consult this forum to find out what's what or sometimes you need to be lucky and try a few.
I think a lot of the Asians will do it - Thai, Chinese but it's no guarantee that they are that kind of massage parlour so you need to tread carefully.
It would be rather embarrassing and frightening to ask for something dirty and a load of angry, disgusted massage therapists throwing you out of somewhere and then shortly afterwards getting a knock on your door from the local police.