How do you ID flags at night?

[Missing Name]

Sadly, you are quite limited, as the game does not model "cruiser rules." IE you can't inspect a ship's cargo.

Close into minimum torpedo range (~400 metres). You should be able to see the flags in light weather and limited clouds (if there is moonlight, of course!). The presence of defensive arms are also a giveaway.

Don't attack lone merchants at night - convoys are more likely to have enemy or enemy "neutral" shipping.