Measuring static thrust is easy - I just use a fish scale tied to the tail. Thrust in flight is normally lower than static thrust, and needs a wind tunnel for accurate measurement.

Consider a fairly clean airplane that can achieve 100 mph (146.7 feet/sec.) straight and level, with a 2 hp engine. Note that one hp = 550 foot-pounds per second.

At 70% propeller efficiency, thrust = (2x550x0.7)/146.7 = 5.25 pounds.

Getting back to my proposal for measuring drag by finding terminal velocity in a vertical dive, with a dead engine, when drag equals weight: If the airplane weighs 7 pounds, it would clearly go faster in a vertical dive than at full power, straight and level.

Its actual speed in vertical dive, neglecting the drag of the stationary prop, would be the square root of (7/5.25), multiplied by the maximum powered speed, straight and level of 100 mph, which equals 115.5 mph.

If the airplane weight is increased from 7 to 10 pounds, vertical dive terminal velocity would increase to 138.1 mph. If 10 pound airplane is extremely clean, and capable of 120 mph straight and level, thrust at 70% prop efficiency would be 4.375 pounds. Vertical dive terminal velocity would increase to 181.4 mph, neglecting the increase expected from the higher Reynolds number.
The height needed to attain this high a terminal velocity would be probably be over 5,000 feet.
Height needed to attain a terminal velocity of 115.5 mph would be about 2,000 feet, do-able with good pilot eyesight.