When you want to know if a magnetic field is present, or if no magnetic field is in the area, you need to use an electromagnetic field meter, or EMF meter. Another name for this device is a gauss meter, after the German physicist Carl Friedrich Gauss. These can be made from a few inexpensive electronics, and use a Hall Effect device that varies its output voltage with a change in magnetic field strength. This voltage is then displayed on a digital voltmeter, where it must then be plugged into an equation to measure the magnetic field strength.
Slide the red wire of a nine-volt battery connector into the upper power bus connection of a solderless bead board, and slide the black wire to the board's ground connection. Do not put a battery in the holder until the device is completed.
Bend the pins of a 5-volt voltage regulator to fit into pins one, two and three of the bead board, then slide the regulator in place. If the regulator's pins will not fit in, solder wires to the pins then slide them into the bead board.
Connect the power bus to pin one of the bead board using a short wire, then connect pin two to the common ground. This will send voltage from the battery into the input of the voltage regulator.
Bend the pins of a Hall Effect device to fit into the bead board, or solder wires to the pins if they are too small. Run a wire between pin one and pin three, then connect pin two to the common ground to connect the Hall Effect device to the circuit.
Set a digital voltmeter to read 20 VDC and slide the red lead into pin three and the black lead into the common ground.
Connect the nine-volt battery into the battery connector to turn the device on. Move the device to an area that is certainly free of magnetic fields, such as an outdoor field away from power lines, to get an initial voltage reading. The voltmeter will read about two and a half volts.
Move the device near a magnetic field, and observe the meter changing. You can use a strong refrigerator magnet, or you can build an electromagnet to generate a field. Multiply the difference between the calibrated zero (about two and a half volts) and the reading by 1,000, and divide by the sensitivity of the Hall Effect device.