The shop I use always shows the old part, and point out where the problem was. Make sure to talk to any repair shop before work is done, and tell them you want to see the old part. They should be willing to give an explanation as to why the part was replaced. Always double check to see a new part has been installed. This won't catch all the problems, but will let the repair shop know you do have some knowledge of what goes on with a car. If something doesn't seem right, call a tow truck, and have the vehicle sent to a different shop.