It's fairly hard to do, but the ones that make an effort will generally at least have the Fair Trade logo. (UTZ is an weak industry-initiated alternative that does not cut it, as I understand it.)
It's all of them.
Tony Chocolonely's whole image is to be as fairtrade as possible and even they admit that they can not guarantee no forced labour was used in their production chain.
They could easily insource their entire operation in order to guarantee slave free chocolate, but you would end up with very expensive chocolate. That is probably why they dropped their "100 percent slave free" slogan. They still need to make a profit and at the moment you can only do that with slave labour.
Yes, looking for fair trade labelled products is important, and I also believe it is important to look for the International Fairtrade Certification Mark ...
As with any such program, if it is implementing effectively then some companies will chafe at the requirements and try to develop their own fair trade program more to their liking.
There's a John Oliver episode about the chocolate industry, you can check it out on YouTube.
Of course there's no logo saying the chocolate is made with child slavery, I hope you didn't mean to be snarky here.
There are initiatives to try to alleviate the problem, the only example I have in mind right now is Tony's Chocolonely (which explains the problem and their solution on their website) but I'm sure others could come up with other initiatives.
The trends from top companies of chocolate is that they try to do as little as possible to alleviate the problem of child slavery in chocolate production.
It's expensive, easy to bypass, and has little enforcement teeth.
Expensive because you need to pay an inspector (ideally multiple to avoid corruption) to visit these farms.
Easy to bypass because the kids aren't working the fields all the time and there's plenty of warning an inspector is coming.
And the enforcement mechanism is you don't get the label... But like there's plenty of buyers for slave chocolate so that mostly means a minor hit on income.
NGO inspectors are better than nothing, but we should be clear eyed in how much they can solve. The only way to significantly reduce slave labor is harsh penalties on companies that trade in slave goods. That was pretty much the only thing that collapsed the ivory market.
You could buy a large plot of land in Ghana or Nigeria. Start a large cacao plantation. Hire locals to do the labour intensive work for you. And then fly in management from Western European countries to oversee the operation and to make sure that no children set foot on the plantation.
