lift up both back wheels so they spin freely. put car in neutral and take parking brake off. spin passenger side rear wheel and have someone watch from the other side and see if the drivers side rear wheel spins. if both back wheels spin when you turn the passenger rear wheel then you have limited slip. if only that one wheels spins that you are turning then it is an open diff. the passenger side rear wheel is the drive wheel in an open diff. and the main drive wheel in a limited slip diff.

not true. alot of times an open diff will cause the opposite wheel to spin opposite from how you are spinning the wheel. if the other wheel spins the same direction as the wheel you're spinning then you most likely have some form of LSD. However, spider gears that are too tight may cause the other wheel to spin forward as well... the only REAL way is to pull the diff cover and have a look. also just because the other tire doesn't spin doesn't mean you don't have LSD. you could have a very WORN LSD... but it makes all the difference when trying to convert it to LSD (cause then you'd just hafta rebuild it if its a S4) and you can't weld spider gears in a LSD rear end to make it a full locker (like a 2 way LSD) like how you could with an open differential.

Ok heres the deal, I know i have lsd because i can leave on hell of a patch (a real burnout, lol 2 marks). with the ass end off the ground, if i spin the drivers side forward, the passenger side will spin backwards. If its an open differential most likely the passengerside will be the drive side I had a v6 mustang along time ago b4 i got the gt and the drivers side would spin free and the passenger side wouldnt move when off the ground.

The open diff is usually pretty good with sending power to both wheels on a strait launch.

A good way to prove you have an lsd is to go into a rainy parking lot, turn the wheel right, and accelerate firmly from a stop.