A "fast and dirty" tool is a battery powered/portable radio that covers the AM radio broadcast band. Tune to unused frequencies (away from the Talk Radio stations) and listen/wander around the area.
Unless you ground that foil hat, it won't do anything about the electric field ...
I use the Tri-Field meter as a standard part of my field troubleshooting kit (I get at least a half-dozen "HELP!!" calls a year to troubleshoot and cure audio system noise. And magnetic fields are the cause in about half the cases. A normal background level for power-frequency (50-60 Hz) is under about 0.5 milli-Gauss. The Tri-Field has a 0 to 3 mG most-sensitive scale. Generally, fields have to reach several tens of mG to be a serious problem with guitars. In one rehearsal hall I was called to, one end of a stage was described as "unplayable" for electric guitars. Holding the meter at waist level and walking toward that end of the stage saw readings climb to a bit over 70 mG. Moving it upward gave a drop in reading and moving down increased it. At the floor, the reading was over 300 mG! Turned out there was a water pipe under the floor and, using my clamp-on ammeter, I confirmed it was carrying 13 A of power-line current. Strangely, everything in the electrical hookup was completely Code compliant. It turned out that the 13 A was flowing from the N-G bond in the building next door (which had its utility feed from the same power lines in the alley) to the N-G bond in my client's building via the main water line connections to each building. The voltage that drove the current was the normal voltage drop in the neutral of the power line in the alley). The cure (and perfectly legal) was to have a plumber install an insulated union on the water line coming in from the street. My client was happy, needless to say!
Unless there's something seriously wrong in electrical wiring, there is zero net magnetic radiation from Romex. Since the same current flows, but in opposite direction, in the white (neutral) and black (line) wires, their magnetic fields completely cancel each other. So a ferromagnetic sheath would be a waste of money and effort. If there IS a significant external magnetic field coming from Romex, it means there's another return path for the current somewhere else. This could be caused by a "bootleg" ground (where neutral is tied to safety ground) for example. Occasionally, this can happen accidentally when a neutral wire gets pinched in a J-box somewhere. But more often, it's an amateur electrician who sees the same zero volts on neutral and safety ground and assumes they're interchangeable!!
Hi, Bill.....thanks for your comments.
