[Russ Beavis]

The stall torque of a DC brush motor is relatively straightforward to calculate.

stall torque = (drive voltage/motor's winding resistance) * motor's torque constant

drive voltage in Volts
resistance in Ohms
torque constant in Nm per Ampere (which is coincidentally equal to the motor's "speed" constant in V per radian/second which can be derived from the unloaded speed)

Be careful with "drive voltage", you'll need to account for losses in the wiring/drive/battery/connectors.

Russ