Hunting for Sport Differential

[superswiss]

Look for the Drive Select button on the center stack (Google images has photos of the different buttons per model year).

Just so OP doesn't get mislead. As I mentioned above, this is not at all a reliable way of telling if the car has the sport differential. As of MY14, even a base model S4 has the Drive Select button to adjust steering and engine/transmission dynamics. Prior to MY14, there were three options/packages that added the Drive Select button. Adaptive Suspension, Sport Differential and Driver Assistance Package (due to the Dynamic Steering and ACC included in the package). The presence of the Drive Select button would only indicate that the car has one or more of these three options, but it won't tell you which one.