Mine was a FL car without servotronic, only way of telling was when we hot the old one out, though I think if you phone Audi was a spec check it should tell you if it has servotronic.
Never done a rack on an S8, but don't imagine it is much different to the A8, and yes it is one hell of a pig of a job, prepare to have your hands and arms ripped to pieces in 'Satan's Hole' !!!
IIRC, we used the write up on Audipages, which is for LHD models, so you just need to think it through a bit.
It will gradually get worse, at the end with mine I was putting in more PAS fluid than petrol, even got a discount on the stuff from my local motor factors
Can you smell anything? The fluid usually drips on something warm and produces quite a distinctive smell.
Another symptom, you may hear quite a groaning noise on full lock, and if you can get it jacked up and turn on full lock, you may even be able to see the fluid coming out.
It is possible to have the rack reconditioned, but just as cheap to do an exchange with another reconditioned unit.