Measure the resistance between the positive lead and the negative lead of the motor power. The stall (starting) current is the rated voltage (12V) divided by the measured resistance. For instance, if the resistance is 2 Ohms: 12V / 2 Ohms would be 6 Amps. When measuring low resistance, short the test leads together and measure the test lead resistance. Then measure the winding resistance and subtract the measured test lead resistance to get a true(er) winding resistance measurement.