Pull the oil fill plug. If you can see springs inside the differential carrier, it's a Sure Grip.
To check manually:Block the front wheels.On level ground, jack the rear of the car up so that the rear wheels are off the ground.Put the rear of your car up on jackstands (good ones), and make sure that your car is stable.Place the gear se1ecter in "N" for neutral.Disengage the parking brake.Turn one wheel and observe the other wheel.If you have a Sure Grip, turning one wheel will turn the other wheel in the SAME direction.If you do not have a Sure-Grip, the other wheel will turn in the OPPOSITE direction.