Identify targets
 

How do I identify a contact that is not in visual range?
Everything would be sooooooooooooooooooooooooooooooooooooooooooooooooo oooooooooooooooooooooooooooooooooooooooooooooooooo ooooooooooooooo much easier if I knew.

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.

To add to that:

ping frequency
number of deamon lines (suggests number of blades on the screw)

Obviously ping frequency only identifies those that are using it. It won't help finding subs. Unless it's driver is either a noob, or very clever. Also, this is just theoretical, as I used to have this list for SCX/Subcommand with pingfrequencies. But haven't found one for DW. Next best thing is the database editor as a reference. Or maybe it is mentioned in the USNI database. Not sure.

Number of deamon lines isn't conclusive. Since the number of lines also depends on the strength of the signal. But if you can see 7 lines, then you can bet it doesn't have only 3.

It's not much help, but every bit of distinction helps sometimes.

Another thing that might help even if it's a very basic one: If you dive under the thermal layer -if any- and the signal is stronger, you have found a sub running under it.

Civilian units also have normally no EMCON, so you might identify them via ESM, but that depends on how they were scripted in the mission.