On the submarine, there is no way to know when the torpedo tube has reloaded if you are at sublevel 2 or lower.  If you are at level 0 (surfaced) or level 1 (Periscope depth) - then the visual indicator will turn from red to yellow if torpedoes are ready. 

When I submerge to level 2 or lower, the yellow indicator goes red because the tubes are disabled at those depths.  In order to verify torpedo status requires exposing the submarine to the enemy and his weapons at level 1.

There is an audio message alerting when Sonar has been recharged.  There should be an audio message alerting 'Torpedo Ready" when a tube has been reloaded.