The rear brakes on my daughters 2012 s60 T5 need to be replaced. The parts list shows two options for replacement kits, one for 300 mm front brakes or one for 315mm front brakes. Is there a way to tell which her car has other than removing the front disc and measuring?

The pictures for the 300 mm version appear to show a no vented rear disc while the 315 mm appear to show a vented rear disc. Her's is non vented, so is this a reliable indicator that I should choose the 300 mm option?

Can it be ascertained from the VIN code?

 

My experience is that the fronts are typically 286MM and the rear 300 MM. The only time that you're going to hae the 315MM vented rear discs are on the R model.

 

On P2 S60's it's either 286mm or 300mm for the front rotors. Your P3 seems to go with either 300mm or 316mm.
Typically, only the R models and AWD models are specked with the larger front rotors.
...Or you could remove a tire and measure.