ORIGINAL: Briman
but I want to know if you all think it would be safe to shoot,
A very unscientific way of testing would be to put the shotgun in a gun vise or sand bag it into position with a pile of sandbags, tie a string to the trigger, load the gun, then fire it with the string while standing a good distance away from it, or maybe from behind a solid wall or vehicle. Use the heaviest loads you can find that will fit the shotgun, look for any problems like blown primers, split bases, hard extraction etc. If you don't have any problems, and have put enough rounds through it to feel comfortable with it, you should be ok. Also check for positive feeding.
Disassemble it afterward to check for cracks, peening, or other damage.
I've done this with a number of guns- some of them over 100 years old to sort of 'proof' them myself to check for headspace or weak metal problems on rifles. I still have all of my fingers and other appendages.
Take an old tire. Cut a 4" diameter hole through the tread on one side. Then lay the tire flat on the ground, load the gun, put the safety on, and stick the barrel throught the hole as far as necessary to be able to put the butt inside the tire on the side opposite the hole. Make sure the muzzle is pointed down-range. Now tie a 15-foot long string to the trigger, and pull it out straight behind the gun butt/tire. Now go up and GENTLY push the safety off. Go back to the end of the string, pull the string and and fire! This is how I proof-fire the muzzleloaders I build. If the gun bursts, the fragments will be caught by the tire and the ground, except for the parts that fly straight up into the air. You are far enough away that none of these pieces will hit you if the thing is faulty.