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...