Except that if the spider gears are frozen somehow, an "open" differential acts just like a Sure-Grip and you can be fooled.
And, here is the correct way to check:
Place the vehicle in "Park" if it is has an automatic transmission, or in first gear if it has a manual transmission.
Jack the vehicle to lift the two tires--on the axle with the differential you are inspecting--off the ground.
Turn the tire on one of the lifted wheels slowly. Use your hand to turn the tire. If the differential is limited slip, the wheel will be difficult to turn. If the differential is an open differential--also known as a conventional differential--the wheel will turn freely.
-or-
Place the vehicle in neutral.
Turn the tire again. The tire should turn more freely on a limited slip. Also, the opposite tire will turn in the same direction. If your vehicle is equipped with an open differential, the wheel on the other side of the axle will turn in the opposite direction.