I presume the only way to detect a hidden seam sleeve job is by seeing the seam on the inside of the barrels. That's assuming the welding and finishing were perfect. That might take a super careful examination maybe even using one of those bore scopes that let you see a close-up of the barrel wall. How many gunsmiths have one of those expensive toys? I've only seen one in a high precision machineshop's inspection dept. and that was 30 years ago.
nial