Brushless Motors kv rating and testing methods/procedures
Kv or 'velocity constant' is the ratio of rotational velocity to applied voltage, usually specified in rpm per Volt. There are 3 ways to measure it:-


Spin the motor shaft at a known rpm, while measuring the generated
peak voltage with an oscilloscope or peak-reading meter.
Connect the motor to a controller and run it unloaded at
full throttle, while measuring the supply voltage and motor rpm.    
Run the motor at full throttle with different loads, measuring
Volts, Amps and rpm for each load. Then calculate kV by
extrapolating the rpm curve to what it would be at zero current draw.


Rpm can be measured with a tachometer on the shaft, or electrically by the measuring the frequency of one phase terminal. If measured electrically you must divide by the number of magnet poles to get mechanical rpm.

Method 1 has the advantage that you don't need a controller, but driving the motor mechanically can be tricky.

Method 2 is easiest if the motor is already connected to a controller and/or installed in a device. However the kV value will be a little low due to voltage drop in the motor's internal resistance, which reduces the effective applied voltage.

Method 3 takes into account the combined characteristics of motor + controller, and also provides enough data to determine the other motor constants, Rm ('dynamic' resistance) and Io (no-load current). 

With Kv, Rm and Io characterized you can calculate the motor's performance under any load.