How to tell if turbos need replacing in as S4?

[imported_druggie]

Appart from the tell tale noisy turbo bearings , how else can you tell if the turbos are faulty or need replacing?
What checks can the workshops perform to tell one way or the other?

 

[imported_ZeroK66]

if the car is not holding boost... boost gauge best way to tell that... if it holds boost evenly through the revs then should be fine.

 

[28v6]

Blowing Oily Smoke... Not holding boost, Not boosting at all, noisy.... That's about it really...