That the neat thing, you don't.

Here, for certain industries (might be all but I don't have first hand accounts of that), the contractors must make sure that the companies/freelancers they employ pay their taxes, otherwise, they are on the hook for it.

Do the same. If a company outsource work, they should prove that they pay the same as they would in their region, and if it not, be hit hard by fines and/or jail time.

But one can only dream I guess