With a high-powered rifle, shoot one of the lines in two at the middle. Observe the two ends. The one that sparks the most is the line going back to the generator.
When these power lines carry DC, I believe it is impossible to easily determine the direction of current flow.
For AC lines, it's fairly easy; one times the 60Hz signal. Use two receivers separated by long enough to get 60Hz out of phase. If the speed of light is 186,000 miles per second, and the speed of wave transmission in the lines is on that order, then a 1% phase difference will need a distance of 0.05 x 186,000 miles/second x (1/60th seconds) = 31 miles.
I learned something from the comments on this question: power companies nowadays adjust the phases of their power lines by putting capacitors and inductors in the line. They do this fairly often and their effect is to modify the relative phase. So if you do measure the field at two points near the wire, make sure that there's not a power station in between your measurements. To learn more about this, try googling "power transmission"+shunt+series