This is a simple magnetic flux meter that you can easily build in an afternoon. You could buy a commercial version, but that isn’t any fun. All the schematics and build information can be found on the 5volt website. If you decide to make one, don’t forget to check out the “How I designed the coil” page.
The reason why I designed it is very simple and lays on the fact that if there’s some uncertainty about effects on the humans of the high frequency electromagnetic fields generated by cellular phones, much less uncertainty is about the damage caused by low frequency magnetic field generated by the mains (220/110 Vac) surrounding us (LFMF).
Make your own Magnetic field flux meter