There are numerous reports out there about the Xinjiang region. The UN High Commissioner for Human Rights did a report after a visit to China in 2022. The Helena Kennedy Centre for International Justice at Sheffield Hallam University also has one where it had people on the ground. There is a Global Affairs Canada 2021 study on supply chain risks related to Xinjiang. There is a lot of information out there. Amnesty International and Human Rights Watch have also published reports.
There has been a lot of information garnered from on the ground. There's also technology to determine whether cotton materials with their DNA have come from that region. There is the ability, from outside of China, to do some investigation.