The firing of a torpedo against a surface ship was usally detected visually, by noticing the wake of bubbles the older 'steam' powered torpedoes left.

the steam torpedoes travelled fast, up to 40 knots or more, but they travelled in a straight line. Once the track was seen a ship would turn parrallel to the track(s) - either towards or away - different situations called for different tactics.

Therefore the surface ship had to match the speed of the torpedo to its turning rate. Turning a small ship away from a torpedo spotted 20 or 30 seconds away was not really difficult.

This ability to turn away when spotted meant that steam torpedoes were at quite a disadvantage. Even lumbering merchantmen could turn away from a single torpedo if they had enough time. To compensate subs often fired a spread of many torpedoes - this fan of torpedoes hopefully compensated for errors in the firing solution as well as any last minute evasive turns by the target.

Eventually 'wakeless' electric torpedoes solved the problem, and steam torpedoes were used mostly at night.

Some of the more advanced ships had sonar that could hear torpedoes 'on the fly' - I believe later German surface warships like the Prinz Eugen (sp?) did, but most escorts had to slow down a lot to hear a torpedo. Sound was not a major factor in detecting incoming torpedoes in WWII.