The Thai massage parlours in Southport I've visited in the past ( Post Office Ave and Ban Chiang) have all had the same girls working there month after month (one even had a UK passport), so have always assumed they were legal and doing this voluntarily.
Since moving to Liverpool Chinese parlours, tho the girl who appears to manage the place (Kelly at Sakura and Coco at Pink Peony) the other girls seem to be there a week or so.
How can you tell they aren't being trafficked?
Its very hard.
The problem is that Chinese women are often complicit in their own exploitation - take the Morecambe cockle pickers, they paid a fortune to be smuggled into the country in containers only to end up as debt-bonded slaves. Language problems, the need to service the debt, and fear of the smugglers stopped them complaining
Chinese women tend to come in via various routes:
smuggled in via container
on tourist / student visas (on which its illegal for them to work)
illegal via Ireland (Irish visas are easier to get than UK ones)
Many of them, especially the older ones seem to be here willingly, but the reality is that often they're paying off a debt-bond - and know that complaining isn't safe. What you've got to ask yourself when you meet a Chinese girl is "how the fuck did she get here"?
Its unrealistic to expect an uneducated girl with no English language from Zhejiang, or Guangong, or Sichuan to have raised the cash to have flown into the UK, and then organised herself to get to a serviced flat in Blackpool, Preston, Leeds or wherever............The truth is someone is organising and financing them and inevitably that organisation and finance comes from chinese organised crime.
By definition the girls are working illegally (unless they have UK residence status through marriage or parents) and are scared of telling anyone the truth. They're between a rock and a hard place: getting caught, chucked out and still having to service the debt -or carry on working, suffer the abuse and brutality of the minders and hope it comes to an end.
Often though the girls are tricked into sex work..........they get a paid flight to the UK on the belief they'll be working in a restaurant, only to find that "noodle bar girl" has a hidden meaning. They arrive, in debt, and have to fuck to survive.
Or you'll get cases like the one I've reported before: a girl flies legitimately into Heathrow on what she thinks is a holiday but the girl she's supposed to meet isn't there. Instead she gets befriended by a pimps agent who offers her a bed and a tour of London. Next day she ends up at an illegal gambling den, doesn't understand the exchange rate, is helped rack up a debt of thousands..........day after she forced to start repaying her debt by working in brothels around the country.
In short its damned hard to tell at times............all you can say for sure is that the older the girl is, then the less likely she is to be forced. Also in Liverpool (which is what you asked about) at least some of the girls are actually British - remember there has been a Chinese population there since the 1700's. However even then there's a good chance the finance comes from Triad-type gangs.