While PM is correct, here's the gist of it:
1. when vehicle is on a lift, spin the driveshaft and watch the wheels. The two rear wheels will behave differently depending on if you have an open or LSD.
2. There is a stamped "S" next to the ratio tag, (located on the bottom edge on the right side looking at the car from the back to front) and a white, painted "S" located on the top of the diff.