There are lots of ways of checking on other vessels for possible collision. The simplest method is to watch the angle between you and the other vessel. If the angle remains the same, you have a collision course. You can do the same thing with radar, by putting the variable range marker on the target. If the target continues down that VRM line, you have a problem.

Or, if you have ARPA-equipped radar - most modern radars have this feature - you can let the radar calculate the crossing vectors. That's what our Furuno 2117 radar is doing in the photo above. We've got two ships heading for us, one to port and one to starboard. This is not the sort of position in which you can make an error.

However, ARPA is not foolproof. Here are some of the risk factors to keep an eye on:

All of which adds up to this. ARPA is a great tool, but keep a visual check on the bearings between you and the other vessel, as a back up.

For lots more on avoiding collisions, see Practical Seamanship.