I tried this method and it works, but probably only good for measuring relative thrust between battery/prop combinations.

-Assemble the plane and tie a loop of string around the tail. Hang it vertically from a digital fish scale.

-Zero the scale, making the weight of the plane ZERO.

-Run the motor at full speed and record the thrust from the scale.

The plane may tend to twist, but a helper can steady it without pulling down on the plane (like the old butcher used to do when weighing meat for your Mom)

I tried it and it seems to show what prop has the best thrust on a certain battery/motor combination.

I wouldn't want to try it on something like George and Dave's TU-4 Bull!