Look at the background: trees, buoys or any other fixed feature. If the other boat appears to be moving backward against the background, you will pass in front of it; if it's moving forward, it will pass you. If the other boat appears steady against the background, you are on a collision course and should probably take action, or at least keep a close eye out.

Not only useful to avoid accidents: when racing sailboats, you often want to run very close to your competitor to steal their air, especially at key points like turns. Watching the background helps you precisely figure your approach from a long way away.

It can also help you tell whether you will beat that locomotive through the railroad crossing.