Tony Chocoloney!
Literally the only one to my knowledge that actually goes out to ensure they're not just being lied to. They are also open about the fact that even though they do their best, they can't absolutely guarantee they're 100% slave free.
Orgs like "fair trade" for example just take the word of producers. It's happened that a tv crew went out to visit one of their farms only to find child slaves at work.
It's a weird old concept is what I guess