Generally warships have a higher revolution sound whereas the merchants are slower sounding......generally.
If you are picking up a definite sound but the notepad in the upper left isn't indicating you should be able to manually force the issue by clicking on the little triangle on that notepad and it will change from 'merchant' to 'warship' to 'unknown' and back again with each click.
I've done this to help identify a target and usually (but not always), if I force the identification it will result in the sonar operator responding and giving a bearing and other info.
Hope this helps.
