I'm going to start buying a used mark 3.5 245 and I want to be sure the car will have the VAQ diff. If there isn't a way to confirm this via paperwork, can you visibly see the unit behind the wheel when the car is on the ground? I doubt many dealers would be able to tell me, honestly or otherwise! 

I think all 245's had the VAQ fitted as standard?

 

Still, a dealer should be able to pull up the original build spec, I believe PR-1Y2 is the VAQ diff

 

If not, try using OBDeleven or VCDS, if you can access module 32 - Differential Locks, then it certainly has a VAQ fitted 

Its on the UK drivers side of the gearbox..so the driveshaft should appear shorter than one without the VAQ unit....basically another ad-on box between the gearbox case & driveshaft flange on the drivers side..