Perhaps there is a case for the punter carrying a card with an appropriate question, in Romanian* asking the girl if she is doing sex work voluntarily...
* or Polish, Hungarian, Mandarin etc as appropriate.
This is a great solution. I've seen sex worker's rights posters in the Soho walkups. This would basically just be a translation of this. Or just a simple question or two as you propose would suffice. I would recommend keeping it on your punting phone rather than a printed card. And no, I don't think you would be at any risk of bodily harm for showing it. If there was a pimp, he's keen to make money, not draw the police to his flat.
I've also gotten the personal contacts for Chinese girls and I've been able to get their background stories, I can see that they're getting out and doing touristy things in the UK, and I can also see when they make it back to their home country safely. But most people won't be able to do this. This is for Chinese places, which I have been to many of, and looked in detail, but have not found any type of coercion or forced labor.
The only place I've ever reported was Romanian, but I pretty much avoid these places at this point. If you were to visit, using the translated question's in Romanian would be best. Maybe somebody here could get their personal contact info, but I'm sure I couldn't.
One legislative solution might be to require a multilingual sex workers' rights poster in the flat/room.