How to measure tonearm effective mass

Some of us who use high or low compliance cartridges fret about mating them with tonearms of low or high effective mass, respectively. Most of us rely upon data supplied by some manufacturers to specify the effective mass of their tonearms, but many manufacturers do not even supply such data. Does anyone know a simple and relatively accurate method for determining effective mass? We know what "effective mass" is; we want to know how to measure it.