How did real captains investigate why their torpedoes were duds?

[Drakken]

I have a question on my mind lately, which I cannot find an answer in my searches. As we know, there are many types of failures with torpedos on a hunt:

- Pre-trigger to explosion
- Precociously going out of steam or fuel OTW
- Hitting the target without explosion, or at a too steep angle to detonate
- Missing the target due to human error
- Going under keel depth because of faulty settings

If there is a malfunction or a mechanical glitch, like during the early war with the TII G7e torpedo, obviously a captain would like to know why to report the issues both to BdU and Ordinance.

For us, it is easy to know why: We press F6 and we can see if a torpedo passes under the keel or hits without an explosion.

However, how were real U-boat commanders able to deduct which type of failure was the cause of a torpedo failure? Obviously, the target not exploding was a big indicator in itself, but what other indicators did they have in hand to find the reasons why and report them back to BdU?