Magnetization in measured in several ways. In the most simple way, you can measure the mass of an object, then apply a known magnetic field and see how much the mass changes. Through the change in mass you can then determine the magnetic field strength. This is an old way to go about this process.

You can also measure magnetic field by passing a magnet near an electrical conductor (say, for example, passing a magnet through a coil of copper wire), and an electrical current will be generated and can be measured.

In modern electronic devices, we measure the magnetic field using a Hall effect sensor. This is a special device that is based on the Hall effect; the voltage of the device depends on the strength of the magnetic field, so by measuring the voltage you can determine the magnetic field strength.

The most sensitive probes of magnetization use a superconducting quantum interference device, or SQUID. These devices need to be cooled to very low temperatures (usually liquid helium or liquid nitrogen), and are extremely sensitive to even the smallest changes in magnetic field.

The strength of a magnetic field is most easily measured by the amount of electrical current it can induce in a loop of moving wire.

The Earth's magnetic field comes from some form of electrical current activity inside of its molten outer core. Exactly how this happens, or why it seems to be disappearing, is unknown and under study. We do know that the Earth's magnetic field does get stronger or weaker with time, and even reverses direction, so what is happening to that it is nothing new. We just don't know what is causing it.