Ships generally have to broadcast a bunch of stuff, like name, from where to where, number of crew etc. constantly while at sea, so all anyone really needs is the ability to receive broadcasts and they can receive those messages and figure out which ships to hit. If a bunch of ships are sending their info but this one isn't you can also assume that that ship is trying to do shifty things so. Besides if they have a lot of people looking at it they can track ships from further away than just right off their coast.