Let’s say you have some spare cash after the holidays (maybe from an unwanted gift you returned) and would like to treat yourself to a designer handbag — perhaps by Louis Vuitton, Chanel or Coach. You could go to one of their boutiques or department store counters. But a growing number of people looking for bargains will shop for these goods through online discounters, or buy them on the secondary market, online or at consignment shops. That’s a bit of a gamble, though, because you can’t be sure you’re getting the real thing.

For example, in the course of reporting the story, Luxury For Less: New Web Sites And Shops Offer Gently Used Designer Goods, Forbes contributor Wendy Goffe bought the handbag pictured above at the San Francisco consignment shop, Cris. The bag says Louis Vuitton on the hardware, and in various spots on the checked pattern. But is it for real?

Cris Zander, owner of the store that bears her name, wouldn’t vouch for the bag, but offered Goffe a full refund of the $500 she spent if it turned out to be a counterfeit. (A comparable Louis Vuitton bag, bought new, would have cost several times that amount.)

We went right to the source, inviting Louis Vuitton to participate in a video at their Fifth Avenue store. Our proposal: have a representative of Louis Vuitton put the bag side by side with the ones in the store, and show us, feature for feature, why it is or is not the real thing.

Our requests to four different Louis Vuitton representatives, over the course of more than a month, went into a black hole. (We did the video anyway, as you can see below.)

Attempts to interview other luxury manufacturers about how to spot a fake of their brand were similarly unsuccessful. Coach did not reply. A representative from Kate Spade said no one was available. Goyard passed. And Chanel sent this statement through a spokesperson: “Authentic Chanel products are only available at Chanel boutiques and authorized dealers.” When we objected that it obviously wasn’t true, the same spokesperson replied by email: “Unfortunately, Chanel has no further comment. Thank you for your understanding.”

Sure, counterfeiting is rampant, but a strong resale market is one sign of a brand’s strength. So it’s too bad companies refuse to help consumers — or talk to the press.

Yes, there’s a small handful of third-party companies that provide authentication services, but proceed with caution. A call to one of them indicated that the phone had been disconnected. The company’s website required an upfront payment before we could get any information. And there was no indication who was running it, who would be doing the work, and what made that person qualified to offer an opinion.

From the consumer’s perspective, there’s surely a need for these services, yet very few businesses have seized the opportunity. The reason, says Susan Scafidi, who heads the Fashion Law Institute at Fordham Law School and writes the Counterfeit Chic blog, Is that “there’s a huge liability if you get it wrong either way.” Designers actively enforce their trademarks, and don’t want a fake identified as the real thing. Likewise, if someone is trying to sell the real thing and it’s wrongly identified as a fake, they too, could sue. All such lawsuits can be costly to defend.

What’s more, if a fake is good, it can be hard for even the company to distinguish it from the real thing, though they have covert, as well as overt, ways of doing that, Scafidi says. For example, the number of stitches per inch in a seam may be a trade secret, and with items like Coach bags that have serial numbers, they can easily tell if it’s for real.

Meanwhile, the proliferation of online sites makes it easier than ever to get snookered. They have professional sounding names; include pictures that look like the real thing (those photos may even violate copyright rules); and price merchandise so it looks like they’re discounting the real thing, rather than overcharging for a knockoff. As with any online vendor you’ll want to consider how long they’ve been in business; how many previous customers there are; and be suspicious if the seller has plenty of stock in an item that’s sold out everywhere else. Good luck getting them to take things back, or expect them to charge a huge restocking fee for the privilege.

You can’t necessarily believe what they say about where something came from either. “It was a gift,” is all too common an explanation. And though you might want to believe that something fell off a truck or went out the back door of the factory at night, these instances are rare, Scafidi says. Don’t be fooled by the presence of a hang tag, either; anyone who can counterfeit a handbag can fake that, too.

What’s a fashionista to do? Train your eye to spot a counterfeit by looking at the real thing in person – at a boutique or an authorized dealer, Scafidi advises. Then, before you buy a bag from another source, give it the once over. Here are the features she recommends you consider.

For “How To Spot A Fake Louis Vuitton,” click here.

Materials. Not all designer bags are made of leather. For example, Louis Vuitton is coated canvas with leather trim. If the trim is supposed to be leather, it should feel dry — not oily, slippery or sticky. The hardware should be heavy — not hollow. If it’s imprinted with the designer’s name, make sure it’s supposed to be.

Workmanship. The stitching should be perfectly even, with no loose threads or back-and-forth stitching at the end of a seam – that’s a sign of sloppy construction. Check the seams for matching–whether of the quilting on a Chanel bag, or the pattern on a Louis Vuitton. A company like Louis Vuitton, which values its logo, wouldn’t divide the letters in a seam. And where the pattern appears on either side of the seam, it should match precisely.

Lining. Counterfeiters rarely have a good view of the inside of a bag. Typically they are working from photographs, which tend to distort color. So if you have gone to a boutique and seen the color in person, you immediately have an advantage. It should be precisely the same shade as the real thing–not a close approximation.

Pockets. Here too, it’s easy for counterfeiters who are working off photographs, rather than copying the real thing, to get it wrong. Scafidi has a fake Chanel tote on which copyists omitted the pocket on the back, and a Coach on which they included the the back pocket but added a zipper.

Place of manufacture. Some counterfeiters routinely mark Louis Vuitton knockoffs “Made in France.” But as it happens, at least one line was made in Spain. An example: a collaboration between Louis Vuitton’s creative director Marc Jacobs, and the artist Takashi Murakami featuring cherry blossoms with whimsical smiley-face centers against the company’s classic brown toile background.

Misspellings. In her collection of counterfeit bags Scafidi has a Coach labeled “eatherware” (the “L” in the word is omitted). Goyard counterfeiters made an even more egregious error, spelling the company’s name “Gooyar” on both the printed canvas of the bag and its dust cover. That’s a dead giveaway.

Based on four of these criteria, Scafadi questioned the authenticity of Goffe’s Louis Vuitton bag, though she couldn’t say for sure whether it was real or fake. The color of the lining isn’t true to the original, she noted; the handles don’t feel like leather; the pattern matching on the seams isn’t perfect; and there is back-and-forth stitching where the seam ends under the tab.

We sent the bag back to Goffe, who made a separate attempt to authenticate it. Without disclosing that she writes for Forbes, she took the bag to the Louis Vuitton store in Seattle and asked if they could repair a scuff on the binding, figuring that they wouldn’t touch a knockoff. The saleswoman looked at it closely and said it was the real deal–an older style that is no longer made.

“She inspected it closely (inside, outside, the lining and in the pockets),” Goffe reported, including the serial number pressed into the felt just above the pocket which would indicate where and when the bag was made. “It was worn off so she couldn’t read it clearly, but based on the overall wear on the bag, the wear on the serial number was normal.” Ordinarily, Louis Vuitton could have redone the piping for $175 but not in this case because the vinyl was cracked, creating the risk of further cracking when they re-piped the seams.

The plot thickened. Could it be that the previous owner had the bag repaired, but not by Louis Vuitton (given how much they charge for those services)? That would explain why the handles weren’t made of the same material as the leather tab at the end of the zipper. In an older bag, with leather handles, they would have shown wear. So maybe the owner had the handles replaced with synthetic ones, which are more durable.

All this suggested that the bag was not a counterfeit–instead call it “custom modified.” Oh the places it might have been before landing in the San Francisco consignment shop. And if only it could talk!

Addendum: Although the bag can’t speak for itself, three days after this article was published Sally A. Carlson, an executive recruiter with Equinox Search in San Francisco, contacted us to speak for it. Carlson, who says she consigned the bag to Zander late last year, wrote in an e-mail that she bought the bag at the Louis Vuitton boutique on Union Square in San Francisco in 2007 and that it has never been altered. She also sent us a receipt showing that she paid $1,120 for it.