I credit VIPER Paranormal for this one. Radio Shack sells wiretaps with a suction cup. If you take that and plug it into a mic jack of a digital recorder and headphones into that jack. Once you press record you can listen to the electromagnetic fields that appliances gives off. You can also get readings without turning on the recorder, but if you take notes and have second recorder with you, you can log what sounds like what.
The first time I used this was at the Fayetteville Hospital during our second investigation. I started getting hits from a column in the center of the room, which the hospital is completely dead. There's no power in the place. We used it again at Lawrenceburg residential home. During our investigation we found a lot of high EMF all over the place. Sometimes you could just a couple of feet from the wall and still hear a buzzing sound. Phones have a buzzing sound, but one had the sound of internet dial-up.
The only draw back is you can't do EVPs unless you take out the wiretap.