Can you see the tops of the existing front brake pads? Stock "S" pads have 2 counter balance type things sticking up out of the top of the pad, while the stock base pads do not, like this:
986 S front brake pad:
986 Base front brake pad:
Note this is not 100% because you can get front "S" pads that do not have the front weight things.
I believe the back pads are the same between the base and S.
You could also look at the front rotors - I think if you swap to the S brakes you need to go to the S rotors as well, all though I am not 100% on that. But if you did have the bigger cross drilled fronts that would be an easy way to tell as well.
So these are not definitive but are two quick ways to tell if you have S front brakes.
Steve