I think that the German raiders generally gathered info from various Allied merchantmen they had captured, etc, which assisted them in misidentifying themselves when challenged. They would pretend to be a Dutch ship or a Spanish ship or something like that, and they might have got hold of various codes to help them in that pretense.

So how do you decide if what they're telling you is real or not?

Do you deliberately sink a ship which you are not sure of the identity of when it may be a neutral merchantman or even a ship that's on your side in the war?

It would be a very tricky business, wouldn't it?

If you did mistakenly sink an allied or neutral merchantman which you thought was a disguised raider due to the fact that you didn't dare approach it, that would also be grounds for court martial, I think...but it would be safer, of course, for your own vessel than closely approaching a possible raider with concealed weapons.

Either way it's a very tricky situation.