https://www.theguardian.com/technology/2012/feb/20/foxconn-r...
The title oddly enough is "Apple faces its 'Nike moment' over working conditions in Chinese factories" from 2012.
One challenge here is that the host government (local, provincial, or national) is often complicit and might not agree to allowing such inspections. In that case, you simply have to be ready to walk away.
Such a strange situation, where a company has to walk away from letting people work for them because they can't verify their working conditions are good enough. I suppose these are the tricky side-effects of being an international corporation though.