Examples are power lines, building wiring, transformers , etc.
The best way to determine the levels of the magnetic fields in an area is to have a survey done. Some utility companies will do a preliminary survey at little or no cost. An alternate is to purchase or rent a field meter and measure the area in question. When you have the survey performed, make sure the monitors in the area are switched off so you can get a true reading of the background fields. Magnetic fields are a direct result of current, or the use of electricity.