A submarine should always be worried about torpedoes.
The easiest way to tell a torpedo from other contacts on sonar is to look at the TIW bearing at the moment of the report. The launching platform might also be there, but at least you know it's one of the two. Torps also tend to be louder, and can often be IDed at DEMON if you assume a TPK of 30 and their speed comes out to 40+ knots or so. Torpedoes in circle mode will leave a visible doppler shift in the narrowband and might leave a squiggly pattern in broadband history. If you use LWAMI 3.11 (or later), torpedoes do not share their 2nd frequency line with ships/subs so they can be rapidly classified.
ASW torpedoes will almost always be active homing, so one way to tell is the ping pattern, especially for torpedoes in circle mode. A circle mode torpedo while searching will only ping for a few seconds until it turns away (3 or so pings), but once it acquires the pings will be continuous. A snaking torpedo can do the same if you are on the edge of its acquisition cone. Pinging rate will also accelerate as range to target decreases, which implies that the weapon has acquired a target. A circling torp that stops making the squiggle pattern or doppler shift has likely acquired something. Rapid bearing change, unless the torpedo is very close, means the torpedo is not on an intercept course for you.
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