There may be some proof marks on the underside of the barrels hidden from view by the stock.
It's not hard to remove the barrels, withdraw the ramrod, set the hammers in half cock, withdraw the pin that goes through the fore arm and rotate the barrels upward and out of the 'hooks' on the breech.
It would also be a good thing to insert the ramrod down both bores and check to make sure it is unloaded, many of these old guns are still loaded and if it's been stored in a dry location the powder can still be viable.
Pretty good looking old piece for the era it's from. Unless you can find some stampings on the bottom, as Griz said, or the name on the lock, if there is one (but rarely means anything more than who built the lock), it's entirely possible and perhaps probable the maker will never be known.