A dedicated, authorized investigator can probably work the difference out really quickly (literally in seconds in most cases, I would imagine, as soon as they speak to the "slave"), but, by contrast, there's pretty much no data we can obtain in this debate (other than a report by an authorized investigator) that will allow us to tell the difference from here.
For better or worse, paying for sex is illegal in most places regardless of consent. The law does not have a carve out for consensual slavery either. The state can decide any consent to participate in an illegal arrangement was cohersed.
> paying for sex is illegal in most places regardless of consent
Surprisingly, it isn't in most places: criminalising it is the "Nordic model" and not widespread. It's things around prostitution that are criminalised (advertising, running a brothel, pimping etc)