Just like any other service providers i.e.. roofers, doctors, furniture
mover, etc. there are good ones and bad ones. The best chance of getting a
square deal is to be an informed consumer. In the case of auto repair it
means keeping abreast of automotive trends and taking the time to learn some
things about the mechanical operation of your car. Next,  good references
are one way to find the better service shops.  Ask around at work and among
friends. Just about everyone has had to have their car worked on at some
time or another, and those people can tell you if they had a good experience
or not. There are internet sites that post various certifications maintained
by some shops (www.asecert.org), but credentials are not always what they
are meant to be.  It helps to be a good judge of character, but not many
people are. Use your common sense and look around any repair shop you are
considering for servicing your car. If it looks clean and the techs look
professional, then you have a better chance of getting good service. Always
be frank with the service manager and let him know up front what kind of
treatment you expect, but be respectful and courteous. Always get an
estimate, with the repairs contingent on your approval. Avoid shops that can
not or will not give estimates. Keep in mind that no one wants to work for
free so even an estimate may cost you something. On expensive repairs that
approach the price of your cars actual value, it is usually advisable to get
a second opinion. Good communication is often the key to a successful
service experience.

Hope this helps.