It could be either. A quick visual inspection should tell you. I believe the
sensors are triggered when pad thickness gets down to 2 millimetres, so look
and see which ones look really low in thickness.

As regards supply of the sensors, I believe these are Porsche dealer only
and cost around £10 a corner, but I stand to be corrected on this ! The pads
can be bought from the likes of German Swedish and French, Euro Car Parts
etc. If the S2 has brakes similar to my '86 Lux, you'll ideally need a set
of new pins, retaining springs etc and these cost around £35 incl VAT an
axle set from Porsche - not sure if this kit can be got from non-dealer
suppliers.

Hope this helps.

Squibby