If it's a factory Torsen diff, then it won't work if both wheels are off the ground. Read this and you'll understand why...

http://auto.howstuffworks.com/differential8.htm

You'd need to have one wheel on the ground and the other in the air, then try to move it (like remove/tighen lugs) and to verify if a Torsen diff is installed. If the diff is open, it will spin when you apply torque to tighten or loosen the lugs. If you have a (torsen) diff, then it won't move.