There is a merchant recognition section in the target book that you see in periscope or UZO view. Click the ^button on the manual and you should see it. You can also click on the word ship on the targeting details if you have auto identify enabled. this will bring up the ship details.
There is a mod you can get to show where to taget a ship. It's called FULL VULNERABILITY MANUAL MOD and can be found on the downloads page of the subsim website. Remeber to kick in a donation to subsim as thanks for the hard work and to keep it running
