Thrust test jig
Propellers are complicated, with different diameters, pitches, chords, twists, and airfoils. At the same time, motor power output is a figure that's dependent on the propeller's resistance to rotation, on the KV of the motor, and on the voltage (throttle and number of cells in serial) applied.
Short-cutting all these variables and arriving at a practical thrust : throttle curve using a testing apparatus is how progress was made in prop aerodynamics up until very recently: most of the practical engineering equations in question are derived from physical testing, not fundamentals & modelling. It's still the gold standard.
This can be done in several ways without the airflow influencing the scale:
 Pulling Down
A square frame is created and rested (from the inside) on the scale. A nose projection is mounted to the bottom, and the motor and prop is put on that nose, with airflow directed upwards. Zero the scale, and you've corrected for the weight of the jig, motor, and prop. Turn the motor on and the scale should show the exact amount of thrust.
 Right Angle
This is more complex, but has the advantage that it can measure thrust without testing the capacity of the scale with added jig weight, and it's a more accurate representation of parasitic drag. Two poles mounted at 90 degrees are placed on a supportive hinge, with the end of one pushing down on the scale, and the other gripping the prop, oriented horizontally. As the prop pushes forward, it pushes the other arm down onto the scale.
If one wants to model parasitic drag effects, one can optionally mount the entire plane on the arm, as long as one ensures that the distance of the motor shaft to the hinge is equal to the distance of the scale-pushing nob to the hinge.
 Commercial Products
 Power analyzer
Equipping a power analyzer while the motor is on the jig will give extremely detailed information on the voltage, amperage, actual RPM, and efficiency of your setup.