I believe the technique taught to military pilots is to continuously move your point of focus through a sweep going in a saw-tooth motion up and down and left to right, stopping only momentarily at one point.
As has been said, you can't easily pick out items that have little relative motion (hence possibly on a collision course) and this technique prevents the gaze being fixed and limited in scope. It also prevents the target being in your blind spot.
If you weren't aware of your blind spot, try drawing two crosses on a piece of paper 6" apart. Focus your eye on one cross and steadily move the paper away, at one point the second cross will disappear as it enters your blind spot.