Find the wires going into it and hook up an ohm meter to it...if you get some resistance going through the windings, that's good. If it shows infinite resistance/overload, the windings are burned up/broken. Then hook one lead to one of the wires for the windings and the other to the metal case of the motor...if you get no resistance (the nicer meters will beep when there's no resistance), you're shorted to the case. Do this for both sides of the windings.
Any of the above showing a problem...you need a new motor. If all checks out...then you can take the motor out and direct wire it to the battery for testing.