[Gary Dillard]

There is an easy method, but it's not what you think:

Output torque of a motor is not a value, it's a graph that's a function of speed. At free speed there is no torque; at zero speed you get the stall torque. And unfortunately by applying a fixed voltage you don't get a given speed or output torque - the motor will run at whatever speed correlates to the applied torque. But there is a pretty simple way to plot the curve.

Mount/fix the motor so the shaft is horizontal and up in the air, like up on a table. Put a spool or pulley on the output shaft and attach a string or cord to the spool with a weight on the end. Apply voltage to the motor and determine the speed of the motor using a tachometer or by timing the speed that the weight moves up and dividing by the circumference. The torque is the weight times the radius of the spool. Change the weight and you will have several points on the speed curve - they should be linear (theoretically they are).

Now, if you want something you could do on the robot that correlates to torque, you could measure the current because it is linear with speed as well. But you would still need to know what torque correlates to what current.