How Can a Factory Tell Whether a Recurring Defect Comes from Tool Wear or Material Variation?

We’re seeing a small but persistent increase in dimensional defects on a metal stamping line. The problem appears on more than one shift, but it is not present on every part. The tooling has been in service for a while, and the latest material batch came from a different supplier.

What measurements or checks would help separate tool wear from material variation? I’m interested in practical ways to investigate the process without changing several settings at once and making the cause harder to identify.