It's all about the barrels. If the ribs are tight and the wall thickness hasn't been excessively compromised it'll be a shootable gun.
Make sure it's cocked by opening the gun all the way and then closing . Remove the forend and then the barrel.
Hang the barrels from the knuckle of your left index finger by the hook and tap along the length of them with your right index finger.
Think mallet and tubular bells.
If they ring like church bells they're good; ribs are tight. If they make a discordant clank they're bad, loose ribs. Make sure it isn't the extractor rattling.
(YouTube can probably show you how to perform this test)
Check the barrels to make sure they have good tight chokes. If the chokes are unmolested it's a better than average chance the walls are good too. Look up the tubes from both directions. Breech to muzzle, muzzle to breech. There will be pits. Decide if it's anything you want to shoot. If they're sewer pipes, leave it alone. If they show signs of honing, be very sceptical of wall thickness.
If it looks promising, take a chance on it, but measure the bores before you shoot it.
A couple hours of cleaning can make a neglected old gun look like a completely different gun at no cost, just a little labor.
These can be nice old guns if you find the sound, solid ones.
It's a buyers market. Supply exceeds demand; more-so all the time.
These guys were prolific gunmakers they turned out a massive number of lower grade guns year in and year out but it'll be a lot classier than whatever kind of a Mossberg can you get for 300 bucks.