[KiltedBanshees93GT]

The easiest would be to check the "axle" code on the door tag or the tag on the transmission itself (then just do a search for mustang VIN decoder). This may or may not be accurate depending on whether a previous owner did a gear swap.
The way to verify would be the following:
(reprinted from a post I made w/ the same question, thanks to Rev for the answer)

"Acxtually, It depoends on whether you have a limited slip differential or not. Raise up one side lifting one rear wheel off the ground. With the car in neutral, try to rotate that off the ground wheel. If it turns easilly, you have an open rear differential.

If it won't turn, then you have a limited slip differential. With the limited slip differential, you must raise both rear wheels off the ground. Then each rotation of the (both) rear wheels will turn the drive shaft a certain number of rotations. That number (eg. 3.25) is your rear axle ratio.

Now, if you have an open axle, put one side firmly on the ground with the other side off the ground. Turn the wheel exactly 2 turns and note the number of turns of the drive shaft. That will tell you more or less the rear end ratio.

Rev"