We measure it directly by sweeping the pickup from 20 Hz to 20 KHz, driving it from a constant current source. We then measure the voltage magnitude across the coil and graph the change in voltage versus frequency. The impedance peak can be determined by the point of highest voltage magnitude. We use a function generator, X-Y Chart recorder and an oscilloscope along with some special equipment built in-house.

Calculating resonance can be tough because you must know the capacitance of the pickup, which is difficult or impossible to measure directly. For the record, the formula is Fr=1/2pi * square root of LC.