One option that sidesteps the question slightly is to learn how to do some of the more common maintenance items yourself. Most bike maintenance is actually quite simple when you have the proper tools. What I've found is that whenever I've needed service done, the cost of having a mechanic doing it is almost the same as purchasing everything I need to do it myself. Thereafter, it's much cheaper since I already have the necessary equipment.
To segue back to your question, knowing how to do most of the basic maintenance on your bike makes it much easier to accurately evaluate the advice you receive from the mechanic at your LBS. Equally as important, if you can talk to the mechanic intelligently about components and repair, they're far less likely to even attempt to bullshit you in the first place. You'd be surprised at the difference in service between the complaints "My gears won't shift" and "My rear derailleur won't index properly any more. I've adjusted the barrel shifters and the B-screw, but shifting still isn't reliable. Any ideas?"
As one quick example, I once took my bike in to my closest LBS to swap out some parts I didn't have the tools for. While the mechanic was performing the service, his assistant made a big deal out of checking my chain wear, then told me it was about time to replace it. Of course, the chain had less than 250 miles on it so I knew the claim was bunk on the face of it. And that was, not coincidentally, the last time I've been to that particular shop.