If you use a big screw driver you can place your ear on the handle and touch it different parts of the motor and it works like a stethoscope. Like I said make sure its plenty long so your face isn't to close to get caught in fans or moving belts.
You could place it on the injectors etc.
As for a flush I prefer to drain a quart or two of oil out and refill with diesel and run at idle for around 5 mins. I have used it on every vehicle I have had and it really cleans it out. Some will argue it will remove good build up and cause leaks. I say if that is the case it needed to be replaced anyway and I have great success with it so thats my 2 cents.
'98 Audi A4 1.8T Quattro 5spd