Handing your car keys over to a mechanic can be a gamble. More than 70 percent of all car owners suspect they've been had, and more than 40 percent know they've been overcharged or underserved by a greasemonkey, according to Repair Trust, a network of automotive professionals. That's why we consulted half a dozen professional mechanics to help you dodge common schemes and drive away unscathed.

Ask Around

Referrals are the best way to dodge shifty mechanics. If you don't have a friend who owns a vehicle similar to yours, find one. Sites such as trustmymechanic.com detail common repair ruses and can provide recommendations. If you do use a referral, remind the mechanic who sent you. Mechanics know that word of mouth can boost their bottom line—or drive customers away. On average, angry customers tell twice as many people about their experience as satisfied ones do, reports TARP Worldwide.

Check the Creds

Credentials and continuing-education certificates are important. But look for local flair. Little League sponsorships or bowling-league trophies mean the shop owner can show his face around town. Check that the garage is certified by the National Institute for Automotive Service Excellence (ASE), as well as by the manufacturer, to work on your car. "Different makes have their own certifications, but most are wrapped around ASE," says John Brekke, a Toyota master technician in suburban Denver.

Take Baby Steps

Don't wait until something goes wrong to visit a new mechanic. Give the shop a test drive with routine oil changes, tire rotations, and wheel alignments. You'll see how it treats customers, and a $5 tip helps ensure you're taken care of when a real problem arises.

Keep a Record

Copies of your past service records should be tidy in your glove compartment. Service history is crucial for mechanics—it can prevent redundant work and help a wrench diagnose problems. Organized records also show that you care about your automobile, which will increase a mechanic's respect for you, says Louis Letizia, co-owner of the independent Manhattan East Auto Repair, in New York City.

Be Specific

"Never just say, 'I hear a noise,' and drop off the keys and leave," says Letizia. Mechanics want to have a dialogue. Make notes about the problem, then describe the what, when, and where. For instance, say, "I hear a high-pitched squeal when I accelerate, and then it stops after 30 mph." A good mechanic immediately knows to check your belts.

Deflate Inflation

Ask if the mechanics work on commission. If they do, think twice. Dana TePoel, co-owner of Lake Arbor Auto near Denver, says the most common scam is a $99 brake job that turns into a $590 project due to unnecessary service recommendations. Afterward, get an itemized invoice so you can catch any double billing (like charging for a water pump and a water-pump gasket). And don't be afraid to haggle. A good mechanic will adjust a final bill to ensure repeat business, especially once it goes north of $1,000. It never hurts to ask.