How can you tell when a turbo is knackered???

its currently on the car, ERST engined mkv escort, and have been told the shaft is stalling

the car smokes abit on startup, suspect valve stems, and is currently suffering with a fueling problem but the turbo itself is boosting and is generally quiet in operation

 

does it blow blue smoke when you stick your foot down? best to get a mate to follow you for this one as its difficult to see from in the car

 

no black smoke

BAD fueling prob