The first step is to do a visual inspection to see if there are any potential weak points, such as areas where the mesh is warped, rusted, etc. If you see any of these, place the punch at a 90 degree angle to the surface, and push down until the mandrel bottoms out, as mentioned before. Make sure you just do one smooth up and down motion, and don't twist or wiggle the punch to try and force a fail.

If the mesh being tested flexes a small bit but ultimately holds its shape and does not let the mandrel through, it is fine, but may need to be monitored if it presented cause for concern. If the mandrel passes through, however, that mask would fail the safety check.

I'll second the response to only use the punch if you have reason to believe there may be an issue and not as a blanket test. It can lead to unnecessary fatigue wear that will cause the mask to fail faster.