[Kevin Sevcik]

If you want to determine it experimentally for the larger motors, you'd need a variable voltage or (better) variable current power supply and a high current multimeter. Then you just need to apply a known torque on the motor. The easiest way is to attach a bar to the end of the shaft and put a weight on it. Then you increase the voltage or current until the mass is just balanced 90* to the side. Measure the current and calculate the torque exerted by the weighted bar. And do this for 4-5 points. There's various other things you can do to make your own actual dynamometer to really measure this, but the above procedure works fairly well and is pretty easy.

Note it's only for the large motors. You probably can't reliably accurately load a small motor and stalling them at any current is dangerous besides.