You'd use gaussmeter to measure field strengths—to determine the extent of the fringe field, for example. A gaussmeter relies on the Hall effect. If an electric current (I) flows through a flat conductor in a magnetic field, the magnetic field exerts a transverse force on the moving charge carriers which tends to push them to one side of the conductor. By measuring voltage produced (the Hall voltage VH), the magnetic field strength (B) can be calculated using
VH = IB / net,
where n = the number density of charge carriers, e = electron charge and t = thickness of the flat conductor.