turbos

how do you know if a turbo is on its way out

and what kinda gas milage do they get

Some people say turbos increase gas mileage, others say it gets worse. Really depends on how you drive the car. I can't really tell you, depends on the car, the setup, etc. Here's a quote from a TSI Turbo kit FAQ

"Yes. The kit comes from the factory set to run safely on the application it was intended for. Many of our customers have run their turbo kits for thousands and thousands of miles with great gas mileage (sometimes gas mileage even increases when off boost!), stock like driveability, and power when they need it!"

So it all varies. But whats a few extra bucks at the tank when you can smoke everyone at the track? :P

Not sure how to tell if the turbo is going out. Just check all the piping and make sure there's oil going to the turbo. Make sure intercooler is clean, etc etc.

You won't get better mileage with a turbo. Plus, you will have to run at least 89 octane gas so you will spend more on fuel.
You would realy need to take the turbo apart to know if it is going out. If the blades or the bearing are shot then it will need work. Sometimes they just get oil leaks and then seize due to the owner not noticing.