THE HALL PROBE
The hall probe is device that allows us to measure magnetic flux density. It is small piece of semiconductor, through which a steady current is flowing, by measuring the p.d. across the probe perpendicular to the direction of the current the strength of a B field can be calculated.

THE SEARCH COIL
When there is a fluctuating field a search coil can be more useful, this is just a small flat coil. The coil is attached to a oscilliscope that allows one to measure the change in current and hence B field over time.