RF interference is a complex field, roughly half is radiative ie the interfering signal is picked up directly from the ether or conductive via power lines etc. If a power line is underground and still radiating the soil must be very poorly conductive /dry. The typical unit to use is a spectrum analyser, big bucks for good gear but some not crazy units like the funcube dongle pro are around .
Common sources of RF interference are smpsu, arc welders, internet over power line units and plasma tvs.
Your am radio is what most people use along with detective work, my smpsu was a shocker, S7 noise across a good chunk of the HF spectrum.