Rosso,
A very good point.
It is common practice by instructors and gunsmiths to get a customer / pupil to mount a PROVEN UNLOADED gun and then the instructor / gunsmith to look back along the rib to see the customer eye placement.
What sadly is not a common practice is to do this test in privacy , with snap caps fitted and the instructors hand/finger placed over the muzzle aperture, also making sure the customers fingers are no where near the trigger/s.
Proving the gun is unloaded to all parties concerned , fitting snap caps and temporarily blocking the muzzles ensures that no accidents are likely to happen.