Where do I check torpedo flooding chances?

Start with the torpedo’s individual stats in a detailed ship-data table or the game’s ship information, if that value is shown there. A single number won’t tell you the chance of flooding on every target, though. Torpedo protection and other flooding-related modifiers can change the result, so treat the listed value as a starting point rather than a guaranteed chance in battle.

I’d compare the skill using the torpedoes you actually expect to land and the targets you usually face. Check whether a reliable ship-data source lists the base flooding chance, then account for the skill’s stated effect and target-side reductions. If you can’t find a clearly sourced value for Jäger, it’s better to leave the result uncertain than to judge the skill from a guessed number.