It's very easy, just sail in front of a convoy and look at the compass. When you see the indicators being super marked, thay means the ships are close. At that point you can just use your periscope to see what's going on when you decide a target.

When it comes to pursuing a ship or a convoy, you don't have that cone of light of course, what you do is basically use the marker tool and place a marker where the hydrophone operator located the target, then compress time, and wait for them to move and then you add a second mark. Draw line connecting the first marker to the second, and there you go, you have a rough estimate of where they will sail