YOU DON'T NEED US TO tell you that buying a used car can be a stressful experience.

But there's one way to alleviate some of the fear that you'll drive off in a dud: You could opt for a certified pre-owned vehicle. All certified vehicles are inspected by a manufacturer-trained mechanic and come with extended warranties that are backed by the manufacturer. Many programs even throw in extra services that once were offered only with new vehicles. BMW, for example, provides free roadside assistance.

Not all programs are failsafe, however. While some warranties extend for several years, others last just a few months. And there's always the chance some unscrupulous dealer will try to pass off any old car as an officially certified vehicle.

Here's how to get a good deal.

Delve Into the Details

Some certified pre-owned programs are much more comprehensive than others. You may find an eight-year or 80,000-mile warranty, roadside assistance, and loaner-car reimbursement through one dealer, and a warranty covering just six months or 10,000 miles with no loaner-car coverage through another. For a complete listing of programs, see Edmunds.com.

Prepare to Pay for Peace of Mind

Considering all the perks and assurances that come with a certified pre-owned vehicle, it makes sense that these cars are more expensive than ones that come without a manufacturer's stamp of approval.

How much more can you expect to pay? That will vary by make and model, but the difference tends to increase with luxury cars. According to figures from auto web site Edmunds.com, you could expect to pay about 8 to 10 percent more for a certified vehicle versus an uncertified vehicle from a dealer. The certified vehicle would probably cost your 15 percent more minimum than if you bought it from an individual.

Watch Out for Dealer-Certified Vehicles

There are two types of certified pre-owned vehicles: ones that are backed by the manufacturer and ones that are certified by someone else. Watch out for dealerships and independent used-car lots that simply slap a "certified" sticker on a windshield, then try to pass that vehicle off as having gone through the same checks dictated by a manufacturer's program. Dealer-certified cars tend to have less-stringent inspection policies and shorter extended warranties than manufacturer-certified vehicles.

How can you tell the difference? First, only a branded dealership can sell its own manufacturer-certified vehicles. So if a BMW dealer is trying to sell you a Mercedes or if you're shopping at an independent lot, you can be confident that the vehicle in question is dealer-certified. It's also a good idea to ask to see all the paperwork that guarantees the car is backed up by the manufacturer.

Go Online

Even with certified vehicles, you need to make sure you're getting what's advertised. The fact is, even though all dealerships must make sure that their certified inventory passes the same 100+ point inspection process, there's some wiggle room that's left to the mechanic's judgment. So while one mechanic might think it's fine to certify a car that has been in a small collision but is otherwise running well, another might not.

To protect yourself, make sure you ask for a full maintenance history and inspection report, and run a vehicle history report from CarFax or AutoCheck.