Place a cleaning rod down the bore from the muzzle end and let it come out the chamber end. Screw a loop on the end with a tight fitting patch. Pull it back into the bore a little. Take a marking pin and place a dot mark right at the muzzle edge on the cleaning rod. Pull the rod out until the dot on the rod makes one full rotation. Measure from the muzzle to the dot. If it is 10" then you have a 10" twist if it is 12" you have a 12" twist.