(FORTUNE Magazine) – It's hard to resist the temptation to buy when you see the fancy fakes sidewalk vendors display. But pick the wrong thing, and you will look like a phony--or a fool--to those who know the genuine article.

Sophisticated consumers who are intimate with real luxury goods are drawn to bargain-priced fakes as much as anyone. They justify purchases with a wink, saying that fakes make great gift ideas, even for their rich friends. After all, a well-done copy, like a trompe l'oeil painting, can be amusing to someone who has everything.

Sometimes socialites go to extremes to find exquisitely crafted and authentic-looking fakes. "Society ladies buy faux products like crazy in Turkey," says Kenneth Jay Lane, the New York City costume jeweler whose client list reads like the Who's Who of the world's most elegant women.

Of course, jet setters don't always have to go abroad to find high-quality knockoffs. Sometimes the goods come to them. New York City socialite Nan Kempner, a lifelong connoisseur of haute couture and a regular on the International Best Dressed list for 30 years, says she acquired a slew of "beautifully made copies" of Chanel handbags from a woman who delivered them personally to her Park Avenue duplex.

Kempner, whose frocks cost about as much as a new car, usually eschews imitations. As she points out, you can't send a fake Fendi, St. Laurent, or Louis Vuitton back to the store for free repairs if something goes wrong, as you can with the real thing. But she couldn't resist the mock $80 Chanel bags--real ones go for $1,100 in the store--because the quality of the detailing such as the stitching and the hardware is "just like" that on her originals.

FORTUNE invited Lane and Kempner to rate fakes recently confiscated by New York private detective Dempster Leech. (For the record: Owning fakes for personal use is no crime; only making or selling them is.) We presented the experts with a mixed selection of spurious and real items, and asked for pointers on how to sort out the legit from the hokey.

Lane took on a pile of phonies at his garment center showroom. Many of the baubles he has on display are identical to those that once adorned Jacqueline Kennedy Onassis. (Lane designed nearly 30 pieces of her jewelry collection that went on the block at Sotheby's in April, including the simulated diamond, ruby, emerald, and topaz necklace and ear clips that sold for $80,000.) His harshest complaints were directed at counterfeiters who stamp designer logos, willy-nilly, on styles and colors of merchandise the legitimate companies don't even make. "I've never seen a Chanel handbag that looks like this," Lane declares, holding a funky, bronze-colored fake. "The color is funny, and the tassel is crudely made," he says. "And it smells like a billy goat," he adds, recoiling.

The heap of counterfeits also includes silk baseball jackets blazoned with the unmistakable LV logo. Lane instantly dismisses them on the grounds that Louis Vuitton does not make clothes. And if Louis Vuitton did make clothes, he adds, the fabric would not be this flimsy.

Lane does find an item he likes: a Chanel drawstring bag of the sort counterfeiters sell on Canal Street. Passing a hand over it appreciatively, he says: "The leather is so soft; it feels so expensive. If I saw this one, I wouldn't know for sure."

In her elegant duplex, brimming with antiques and art (the genuine kind, including two Picassos and a Matisse), Kempner inspects each fake as carefully as she would a new haute couture creation, and cheerfully pulls out real designer goods from her collection to make comparisons. This lady knows clothes: She wore her first couture ensemble from Paris when she was only 3. St. Laurent has been her favorite since she made his acquaintance in Paris in the mid-Fifties, when he was Christian Dior's young assistant.

Kempner laughs at the counterfeiters' creative excess. A small suitcase flaunts a Louis Vuitton logo stitched on askew in addition to the LV insignia. "I know this is fake because Louis Vuitton doesn't go to that much trouble to let you know it is authentic," says Kempner. "For some reason, the people who make copies think they have to give you more features than the original."

When Kempner scrutinizes a phony Vuitton vanity case that could probably pass for real to an untrained eye, she complains: "The push button on the lock is wrong; the real one has a clasp you pinch. The color of the border is a shade or so off. And this case is too lightweight. I can barely lift the real one."

From across the room she instantly differentiates look-alike Louis Vuitton handbags sitting side by side: The color is wrong on the fake. Slinging both bags under her arms, Kempner ticks off several reasons the impostor doesn't measure up: "The stitching is crooked, details like the buckle look cheap, and the shoulder strap is too long. These days women want a shorter strap, like the real one has, so they can clutch it tightly under their arm."

The moral? If you want designer goods that don't make you look like a phony or a fool, play it safe and save up for the genuine article. But if you can't resist the notion of saving money by buying a fake, do a thorough study of the real designer's merchandise beforehand, or beg an expert for help. However, Mrs. Kempner and Mr. Lane are available by appointment only.