Your digital camera or smartphone. Take pictures and videos to see what is happening with the gun when firing, or when something is broken you can show the customer a picture. Not just to post on social media (although I guess you can do that too….)
Magnifying glass, to look at firing pin indents, seeing cracks in metal parts, and many other uses. For example when a .22 Rimfire misfires, take a look at the indent in the rim of the case. How large is it? Is it the right shape? How deep is it? Does it look like it is providing enough disruptive energy?