Make yourself seem assertive and in the know
Spend some time reading up about possible reasons your car is having trouble before you even show up for your initial consult. Coming to the mechanic with a list of things you have considered on your own makes you seem self-aware and proactive. A shady mechanic will be less likely to try to pull a fast one on someone that seems informed and self-confident than on someone who shows up with no clue what the problem is.
Ask several blunt questions while making eye contact
After the mechanic has made his or her first assessment of the car and returns either with a guess as to the problem or tells you they will need more time to further explore the problem, as the mechanic how their pricing structure is set-up. Do they charge a flat rate per hour of labor? Do they charge on a per-job basis where each kind of task or repair has it’s own unique price? Where do they order their parts from and how much do they mark them up when they charge you for them? Asking such questions while making direct eye contact lets the mechanic know you are not going to be easily fooled.
Use the Web
Many sites such as yellowpages.com, google based directories and premium websites such as Angie’s list provide reviews on local businesses written by consumers living in your area. Check the same business or auto shop on several websites to ensure a good spread of information. Remember that one or two complaints may be a fluke or only represent one or two people being grumpy because their car legitimately cost them big bucks. However, if the majority of contributors are complaining, it’s a bad sign!
Are there many other customers waiting to have their cars fixed here? If so, ask them if they are return business. A good amount of return business suggests the mechanic is doing something right. If the place is always empty besides you, it’s a bad sign!