Servo Torque
You would need to measure the torque with a strain gauge whilst loaded.
This however is not a REAL practical measure of servo torque......
There are other considerations other than just brute force. Most servos would probably carry up to 3~5 times the load that's advertised, however they cannot do that for very long time. They would also be sluggish, and not up to full advertised acceleration speed....
The other variables are transit time, the current drawn from the battery (power), and finally the phisical limitations of the geartrain.
I'm afraid that you can only use the manufacturer's published torque rating.....I'm not even sure they use the same methods (industry standardized measure) to come to their conclusions.