There are standards for the amount of force the mask must withstand and punch test devices like these [1] to exert them repeatably. Officials punch a few times across the front and the sides, look for failure or severe deformation, and the mask passes for the event.

[1] https://radicalfencing.com/products/rf-pbt-mask-punch-tester

A sharp punch (like a center punch prior to drilling). It is pushed in with a certain force and the mask mesh can only spread so much; there is a groove on the neck of the punch that defines the depth limit. You do this on a few areas of the mask then date the side of the mask to indicate it was tested.

Wouldn't repeated testing (punching etc.) itself weaken the mask?

Yes. That said, if the testing causes the mask to weaken enough to fail, that means it's time to replace the mask.

To add to that, the mask is in some sense "punch tested" nearly every time you use it. Far better for it to fail in a test.

Agree. One punch test is nothing compared to the 20 - 30 times you might take one to the face (sometimes intentionally) in a single bout. Absolutely do not want that thing failing.

last i fenced they also had manufacture dates printed prominently and were considered expired after a few years.

they’re wear items; it is their job to take the damage so you don’t.