I tested some of my motors using this technique: "How To Test Stepper Performance" .
The rating of a motor is supposed to be in oz./in. or ft./lbs. I have also seen them rated in the metric system.
So, a 100 oz. motor will hold 100 oz. hanging off a 1" arm. Or 10 oz off of a 10" arm. Or 8.33 lbs / .0833 ft.
The real specs though are in the torque curve. That is a graph that shows, that at a particular voltage, how much torque the motor puts out as you vary the speed.
Here are some torque curves for an Oriental Motor PK266-2A