An ESC uses a solid state circuit to create a complex PWM waveform to push and sense the three poles of a brushless motor. This is a somewhat complex waveform, and power load - one cannot simply hook up a multimeter between battery and ESC, or between ESC and motor ( and good luck finding one that can affordibly do high voltage, low amperage work without modifying the circuit by adding significantly to battery ISR). A power analyzer specialized to this task will tell you RPM, resistances, current, voltages, and power.
 Thrust test jig
When combined with a thrust test jig, a power analyzer becomes an even more informative tool, as thrust efficiency, and a throttle response curve can be created.