Maybe try a high power shunt resistor connected to the output of a second motor of the same design, with the motor shafts connected. Measure the voltage across the (known) resistance to determine the current through the load, then multiply the current by the voltage to determine power in watts. Since you already have the input power using the method suggested by the others just complete the formula. I would try a number of input voltages and currents. I think your answer would have to be multiplied by two, since there would be losses from both motors.