Obstacle Clearance
Qoute:
if you could see features beyond the one you are looking at, then you are at a height to clear it.
Another way of thinking about it and it's very simple it comes from looking a the threshold on final approach.
If the obstacle is moving upwards in the front windscreen your going into it or in the case of powerlines under them, If the obstacle is constant your flying directly for it and if the obstacle is moving downwards in the windsreen you are flying over it, Clear of it!!
I'm sure anyone with a lowlevel endorsement will be able to shed light on how the technique is taught.