If it's possible to tell, then it should be a criminal act to have sex with a woman being held against her will as a sex slave. In fact it is already.

But if it's possible to tell, then the police should move in, and the women released, shouldn't they? It's not for the general public to do police's investigation work for them.

If it isn't possible to tell, then no-one should be incriminated for buying sex innocently if some time later, possibly years later, after the man has married and started a family, it is discovered that the prostitutes were being forced to sell themselves by some trafficker.