New owner of S80, I have a bad clunk on the curb side front in rough conditions. How do you tell if the struts are bad? The curbside is about .5 Inch lower that the driver side?

 

Check your upper spring seat. You do this by opening the hood and locating the nut on the top of the strut.
Get a 13/16 wrench and try turning the nut. If it turns easy then the spring seat is bad. If it feels rubbery then it is good.

 

I can confirm this. I used this method on the s60 I was buying after noticing some "thunking" when going over bumps. The dealer said "no problem, I'll take care of it". It ended up being about $285 to have the work done, but he did replace the spring seat.