Well you can always pick an enemy harbor and sneak up to it during the day. Pick one without a roving guard ship, like the one on the north east coast of Formosa. You want something that has patrol boats sitting around doing nothing. Unless they detect you, they don't start shooting or move from their spots, so they are a great way to test your visibility.
Move up to it (submerged) until you are about 10nm away. Save your game at this point, so you have the same starting conditions each time.
Surface, then motor straight forward at about 2-3 knots, plain as day. When they finally spot you, make a note of how close you got.
Reload and do it again, only this time set your depth at the decks awash setting. You should be able to get closer before they see you.
Reload and do it again at periscope depth, with your attack scope up and visible.
Then you can compress time and try it again at night if you like. You should see a difference between each method.
(Edit: You can also use an enemy fishing boat/sampan to check this. The next time you encounter one, save your game then try going straight at him surfaced/decks awash/periscope depth. Note the point at which he freaks out and starts making frantic turns to avoid you, then reload and check a different method.)
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