If possible you should be using a mechanic that has been recommended by somebody who knows cars. Even gearheads have their limits and are forced to pay a couple extra bucks every once in a while to do something that's too big with their selection of tools. If they're cool with the guy, then you should be able to trust him. If nobody has any suggestions, have somebody who knows cars come with you when you take your car into the shop. Have the mechanic explain what needs to be done, and if your auto-savvy friend approves, then you're good to go.
If neither of these are options, get multiple opinions. Take your car to the local, family-owned shop, a big chain, and that one that you've seen with the vintage Chevy Camaros tucked away in the back. More times than not, you'll find out who is overcharging and over-fixing. Once you find your mechanic, make sure you stay loyal and treat him or her with respect. With a stronger relationship, you'll not only learn more about your car, but won't have to worry about getting screwed over.