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The proof markings are on the bottom on the barrels so you'll need to remove them from the stock. You'll need to tap out the two or three wedge keys and the barrels will slide off. Usually there will be a number either 4, 6, 8, 9, 10 ,11 ,12 , 14, 16, 20, 24, 25, 26, 28 or 32 these numbers denote gauge. There should be some English proof markings too little triagular patterned numbers and a seal in the center. These proof marks change every couple of years so that will put you in the ball park of what year it was built it. There might be an exact date but I highly doubt it. Like smoothboremurph said get some brass drift and pin punches and some good gunsmith turnscrews so you dont damage anything.