If your Calvins could talk, what would they say? That they are fake, and you were conned?

Private detectives are gazing at rear ends these days to try to spot the difference. Counterfeiting of designer jeans is such a big business that makers of Calvin Klein, Jordache, Gloria Vanderbilt and other high-fashion trousers and T-shirts are spending millions of dollars to nab manufacturers and sellers of the phony products.

Counterfeiting always has had good street value. Witness the success of the phony dollar bill. But designer jeans and Levis manufacturers say they never expected such whopping competition. Last year counterfeiters may have generated sales equal to 10 per cent of the $400 to $500 million wholesale designer jeans market. The crime is trademark infringement and fraud.

"Recently I was on vacation in North and South Carolina, and I saw some counterfeit Calvins in a store," says Joel Sandleman, a vice president of Puritan Fashions, the makers of Calvin Klein jeans and related products. o

"Later I was on an airplane and a young lady in counterfeit jeans walked by. Then I was in a hotel, and I saw a man in counterfeit Calvins.

"We sell what we produce.If these jeans weren't around, we'd sell more." Probably $10 million more last year, company officials have estimated.

Puritan Fashions, which expected to sell more than $100 million in jeans last year, spent $1 million in legal and investigative fees to track down and eliminate the phonies.

A suit filed in Los Angeles claims that Jordache has lost "in excess of $1 million" in nationwide sales to bogus copies. Last summer, Levi Strauss and Co. uncovered a U.S. counterfeiting operation that was producing an estimated 50,000 fake pairs a month. In September, they won a $500,000 settlement in London from the operators of an international ring selling phony Levis abroad.

"In Italy, the No. 1 jean was Levis," says Bud Johns, a Levis spokesman. "The No. 2 brand was counterfeit Levis."

How would you know? Start with the price. Bogus designer jeans may sell for as little as $15, while the real thing usually costs between $38 and $45. Flea markets and out-of-the way outlets are popular unloading points. Sales and discounts offered by legitimate businesses only confuse the matter. So manufacturers are trying to educate the public with new ads about the telling distinctions.

In a pair of bogus jeans, the seams may fall apart after one washing, or the hangtag may not be printed with proper specifications, or the zipper is cheap, the rivets dime-store variety and sometimes, quelle horreur, the counterfeiters can't spell. One batch of fake Calvins had "Calvin Klien" inscribed on the rear pocket.

Not long ago, Sheldon Goldstein, a former Miami Beach policeman, saw three fannies covered with brand new "Calvins" in Miami International Airport.

Trouble was, the stitching looked different. Goldstein, who works for Rocky Pomerance Associates, a private investigative firm hired by Puritan Fashions, sent a pair to the manufacturer for inspection.

The results aren't in, but Goldstein thinks he may have found another counterfeit operation. The 25 detectives working for Pomerance Associates, the largest national private firm investigating the designer crime business, have turned up phony Calvins in almost a dozen states since last spring.

Their clients also include Izod-Lacoste (the alligator people), Jordache, Geoffrey Beene, Yves St. Laurent and Cartier.

"I don't think there will be an end to the problem as long as status symbols are what people want to put their money into," says James Bikoff, secretary and general counsel for Cartier, Inc. "There is a market for people who can't afford the real thing. . . . Some people just don't care."

Last year, Cartier sold about 200,000 watches, starting at $650 each. There were almost as many counterfeits in circulation, selling for as little as $22. "Some people," says Bikoff, "will believe anything and buy anything."

A raid last year in Miami produced hundreds of cartons of shirts bearing the names of Jordache, Calvin Klein, Christian Dior, Diane von Furstenburg, Yves St. Laurent and Givenchy, as well as the heating machines for transferring labels to the shirts. The T-shirts sold for $24.50 a dozen. (The average designer shirt retails for about five times that amount.)

The typical counterfeit ring works like the one uncovered by Pomerance detectives last fall in Florida. The labels for fake Calvins were manufactured by a label-maker in New York, shipped to an innocent relative of one of the cons in Delaware, reshipped to Dallas to a house occupied by a very old lady, who never received the goods because they were diverted en route by the con to Daytona Beach. There the labels were sewn to jeans seconds and irregulars purchased in Georgia.

The finished product was sold for $14 as a Calvin. One of the cons was arrested, pleaded guilty to fraud and was given five years supervised probation. Her partner fled and is wanted on fugitive warrants in several states.

Caveat emptor. The manuafacturers are trying to help you. A new series of Calvin Klein ads, which have been shown in Boston, demonstrate the difference between the fake and the real thing. It's not, however, Brooke Shields' derriere in the spot announcements.

"She's there," says Sandleman, "to sell."