Out at sea, we know that a collision risk exists by noting whether the vessel in question is keeping a steady bearing. We all do this in practice by keeping our heads still and noting whether a ship coming over the horizon stays on top of a stanchion. If it looks like a possibility, we watch her compass bearing as the range closes. Inshore, it’s far easier to note whether the other craft appears steady relative to its distant background. If the bow of this ship stays still in relation the factory chimney behind it, it’s on a steady bearing and she’s going to whack you with her anchor. The geometry of this is hard to prove and it doesn’t work if the other vessel is very close to shore, but otherwise it’s reliable.