Depends .. you can also use ESM, or narrowband sonar. Sometimes other platforms detect and identify targets and sends then into 'the link'.
Most of the time you don't need to know exactly what the target is. You need to know class, and friend or foe. For friend or foe there is one neat trick .. most Russians use 50Hz as base narrowband line .. while most other use 60Hz. As this is usually the first line you detect (on towed array), it's your first clue.
But beware, in some campaign mission, your task is to 'identify' targets .. but you actually must 'mark' them via periscope. No other mean of identification will work.
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