This information will most likely be found in a Responsible Minerals Sourcing Policy, Sustainability Report or Conflict Minerals Report.
For example, after an audit, the company may give the audited supplier a check-list with improvement points (often refered to as a Corrective Action Plan).
If the company has a Modern Slavery statement that is also a place where you might find this information, but the details must be explicitly related to the minerals chain in order to receive a 'Yes'.