Powerlines are one of the biggest accident causes for ag. pilots.
In the country in Oz single wire earth return (SWER)systems are common. As the name says, it use a single wire. Very, very difficult to see.
As others have said, look for the poles, terrain that has a different 'look' to the surroundings etc. Also note buildings. Just about every building & structure in the western world is wired for electricity. It has to get to the building somehow so use the building's presence as a heads up about powerlines.