First, make sure the barrels haven't been cut. The two barrels should be touching at the muzzle. If they're not and there is a filled in gap between the two then they've likely been cut which means you have no choke left.

Next take the barrels off and hang them off your finger by the barrel hook. Give the barrels a flick with your fingernail. They should have a ring to them. If they sound dead then the solder joint holding the barrels together has likely gone bad and the need re-soldering. (big expense). Also look down the rib and make sure it's perfectly straight. If it's "wowed" anywhere along it's length then it has likely come loose sometime in the past and been poorly repaired.

While you have the barrels off look for the gauge/chamber length proof marks. They're located in a flat sided oval on the barrel flats. The first number is the gauge and the second is the chamber length in millimeters. In this case it's 12ga., 76mm (3") chambers. 70mm would be 2 3/4" chambers.

Also check the surface of the barrel hook. This should be smooth and free of any major gouges. Also look for peen marks made by a center punch. This is an old method used by unscrupulous sellers to make a loose hings seem tight.

With the barrels back on but the fore-arm still off, grasp one hand on the grip an the other on the barrels and wiggle. You should not feel any movement or play between the barrels and receiver.
Put the fore-arm back on and open the gun. Again, check for any looseness in the hinge.
Close the gun and look for any gap between the barrel ends and the face of the receiver.

With the gun firmly closed make sure the opening lever is slightly right of center. If it's perfectly centered then the locking mechanism is showing signs of wear. The gun still probably still has lots of life left in it especially if you're not doing and high volume shooting. If it's left of center then the locking mechanism is quite worn and the gun is in need of some major repair.

If all the above checks out then the gun is likely structurally sound and worth considering buying. Always remember with any double gun KEEP THE HINGE PIN AND LOCKING LUGS WELL LUBRICATED. I use a good quality grease but oil works also.