Barrel should be inspected for any obvious defects such as dent, dings and rust. Dents can act as a barrel obstruction, so they are to be taken seroiously. I dropped my CZ Ringneck on the concrete this Summer and it only received minor scratches. I am of the onpinion that dings and dents do not constitute normal wear, but abuse. The bore should be shinny and have "rings" when viewed.
Ringing of the barrels is a good indication that they are properly joined. Also tip the barrels up and down and listen for "sand" sliding. this is solder. Some sliding is o.k., but excessive sliding or if you hear chuncks of solder clanging down the joint you proably have a problem barrel.
ejectors (if any) should be checked. Insert snap caps and see if the ejectors work individually and together. Both snap caps should fly out at the same time. If not the ejectors are not timed.
Barrel thickness is also important. a lot of old doubles have seen better days. Barrels should have some "meat" to them and not appear paper thin at the muzzles. In Michael McIntosh's book SHOTGUN TECHNICANA, Chaper 27, it is stated that a 12 gauge with a nominal bore diameter of .729" is out of proof, "unsafe" at .740" there is more to it and it is rather, well...Technical, but use your best judgement and look for barrels that appear unusually thin.
I use a powerful flashlight to check a gun out. bright light lets you see things that might have skipped your attention, or that someone has tried to cover up. A good inspection of a shotgun should take a 1/2 hour at the least.
Hope this helps
