How do I know if a garage is quoting fair or just guessing?

[jacintocollier]

I've been putting off a proper look at my A4 because every time I ask around I get a different story. One guy says it's the turbo, another says it's just a sensor, and a third wants to charge me a diagnostic fee before he'll even tell me what he thinks. The car is still driving but it's not pulling like it used to and there's a slight whistle that wasn't there before, so I'd rather deal with it now than wait for something to give. A friend told me to find out more about a place that specialises in these cars rather than keep asking random garages, which sounds sensible, but I'm also wary of walking into a sales pitch. Has anyone been through this where you're not even sure what's wrong yet? How did you work out who was actually diagnosing the problem and who was just guessing at your expense? I don't mind paying for a proper look, I just want to know it's a real answer and not a fishing trip.