Lift the rear of the car so both of the wheels are off the ground, and with the transmission in neutral, and the parking brake released, spin one of the wheels. If the rear end is equipped with a limited slip differential, the other wheel will spin in the same direction as the one you are turning. If it is equipped with an open differential the wheel will spin in the opposite direction.
You can also check with only one wheel off of the ground. If the car has an open differential, the wheel will spin relatively easily. If it has a limited slip differential, you will feel a good deal of resistance while spinning the wheel.