You can shine a light down the bores and get an idea, but the only sure way I know is to unbreach it and than look in from the back. I'm in the same state as you, I just bought a Middleton double percussion that looks fantastic on the outside, but who knows what it looks like on the inside. For sure the safest approach is to put it in the cabinet and not shoot it, but if you want to risk it take the barrels from the stock and fire a proof load in each barrel, than examine afterwords and see what you have. You probably will want to change the nipples in any event, Track offers nipples that should fit these older guns, or they can be retapped to a modern size.