So just curious, but what can a gunsmith do that we can't in terms of checking to see if the bore/chamber can handle the pressure of a round?
Cracks? hardness?
How are these tests conducted?
Mostly what a gunsmith offers is training and experience. In most cases, such an inspection won't include any specialized equipment. However barrel wall thickness may be measured.
The gunsmith will also utilize his/her training in firearms history and metallurgy to determine the approximate age of the gun, the barrel manufacturing method, the composition of the steel/alloy, and the action type. This, along with a visual inspection, will assist the Smith in judging whether or not the gun is within a safety margin of error.
The visual inspection and measurements look for clues such as chamber length, chamber and barrel wall thickness, rust, pitting, cracks, dings, bulges, loose ribs or other degradation of solder. The receiver is also scrutinized for looseness or "play", excessive wear, jamming, or any malfunction that could affect the operation or integrity, or safe use of the firearm. Corrosion, cracks, evidence of poor maintenance (infrequent cleaning, build-up), loose or missing parts, or modifications from original build are noted.
Stocks are inspected for cracks, dents & dings, absorption of oil or water, proper adjustment - in short, anything that could cause them to fail during use or that might cause devaluation of your purchase.
Many gunsmiths will remotely test fire the gun in a safety chamber using "test" rounds generating known pressures as a final step if all else looks good.