When you're looking at old guns , particularly 16 and 20 bores, have a chamber gauge handy (connecticutshotgun.com), as many guns made before the mid-1920s have cham- bers too short for modern shells. They can be lengthened, but that's a good bargaining point.
Remove the forend and feel for looseness by holding the barrels tightly and swinging it back and forth. Hold the action and try to move the barrels sideways and up and down to feel for play. Re- joining barrels will make $500 or more evaporate.
These should be shiny, but hang them by the hook on your finger and flick both barrels with a wood dowel or a fingernail on your other hand. They should ring like a bell. Also, look down the bore of the barrel for unusual ripples or bulges.
Be careful with sidelock guns, as their stocks are very thin at the head and tend to crack. Very dark wood at the stock’s head normally means it is oil-soaked and mushy. Replacing the forend with the same grain of wood as the original is virtually impossible.