How to determine if you are on a collision course

When I first started sailing the bay, I was very panicked that I was going to hit every boat in my vicinity.  Then a friend told me about the Nichol's Effect.  This is a law that has saved me many times.  Let's say you are sailing on a port tack and donnot have the right of way over a boat heading across your bow 100 yards away.  Do you need to tack to avoid him?  Not if you use this handy technique.  If there is land behind the boat, use that as your gauge.  Look at the boat and the land behind it.  If the land is moving, you are not on a collision course.  Good news!  Now if the land is moving from bow to stern, the boat will pass in front of you.  Stern to bow and it goes behind you.  If there is no movement of the land as the boat moves, be prepared to take evasive action.  If you are on the ocean with no land in sight, use a stanchion or some other part of your boat, to determine his course (a bearing compass works well too).  When I sail the bay, I assume no one knows the rules of the road (which everyone should know) and I always give everyone else the right of way in questionable situations.  Get out on the water and try out this law and amaze your friends with your sailing acumen.