THE CASE was worthy of Lord Peter Wimsey: to detect the truth beneath layers of artful deception. The evidence, a small gilded fragment from a grandiose Chippendale-style mirror, concealed its origins under a mask of accumulated dust. Was the piece original or a clever fake?

"The design was that of the mid-18th century, and on the wall it certainly looked like the real thing," recalled dealer and sleuth Glenn Randall, a specialist in 17th- 18th-century furniture. "I had to examine it carefully to determine that it was really a wonderful copy of about 1850 to 1900.

"At that time, there wasn't any intent to deceive. But after 100 years, the normal aging of broken chips, grime and oxidation could fool most people."

The clues were evident only to an expert: The copy was made of poplar, rather than the North Sea pine typical of the period in England. Simulated old glass used a century ago has aged differently from the original material. Carving was cut out by machine rather than by hand. And the piece was put together with a synthetic substance unknown in the 18th century, when cabinetmakers used animal glues.

"It takes a really trained person with experience to detect certain kinds of repairs or outright frauds. It's a tricky business with prices so staggering," concedes Clement Conger, curator of the White House and chairman of the Fine Arts Committee for the diplomatic reception rooms at the State Department. On pieces of great value, he seeks the opinion of American furniture dealer Harold Sack in New York.

As the demand for antique furnishings escalates and the supply of original pieces dwindles, deceptions of all sorts are more profitable than ever.

An authentic mirror of the mid-18th century, a fanciful chinoiserie design hanging in Randall's shop, is priced at $15,000. "If the other were original, it would be equal in price," he says. "As an elaborate 19th-century copy, it is worth $1,500 to $2,000 as decoration and nothing as an antique."

Besides complete frauds, there are related practices of altering -- adding or taking something away -- in an effort to increase the value, and the concealment of repairs and replacement of vital parts. A block-front bureau with original feet, for example, could command $35,000, compared with $6,000 to $7,000 with the feet restored.

"You have to know where to look and what to look for," notes Harold Sack, president of Israel Sack, Inc. "Certain types of things are often faked, for instance: block fronts converted from straight fronts and flat-top highboys replaced with more desirable bonnet tops.

"There are certain vulnerable parts of pieces that are frequently repaired -- the rear legs of chairs, the crests and back splats of chairs where people rest their backs, the feet on heavy case pieces that are banged and broken during moves. If you can zero in on these places, like a doctor who knows where to look for weaknesses in an old man, you're halfway home."

Such specialized knowledge is clearly not held by the amateur. But even in a field sown dangerously with mines, there are signals to watch for. Bethesda dealer William Blair described a method for approaching fine 17th- and 18th-century English antiques.

"You have to consider how a piece was used over the years, where and by whom. Certain repairs that are the result of normal wear and tear in the course of usage do not affect the value of a piece. The repair of drawer runners which eventually wear out, or replacement of brasses on early chests, the type with bands that bend at right angles behind and fell off with use, are not considered serious. However cutting off the extended back of a sideboard top to streamline the piece for modern homes would be serious."

As evidence of antiquity, says Blair, "look for shrinkage with age. Over the years the wood of the carcass shrinks, and this will show up in cracked veneers, split sides and moldings that extend slightly beyond the shrinking frame. On inlaid work, peaks and valleys can be felt along the surface. Chair frames become loose as wood shrinks."

Signs of wear should be found in the smooth, polished surfaces on the bottoms of chairs dragged for many years along the floor.

Hidden surfaces reveal methods of construction not apparent in the finished parts of pieces. "Eighteenth-century craftsmen never took pains with any area that didn't meet the eye. These places were left coarse and crude," Blair says. The result of hand planing, a rippling, wavy effect, should be felt on unfinished backs, for example. A smooth, slick surface would reflect machine work.

Another clue is the wide timber no longer available in modern lumber yards. Blair pointed to a piecrust table measuring 31 inches in diameter, one solid piece of wood. "If the top were made of two matched pieces, it would indicate modern construction; an 18th-century cabinetmaker would never finish a piece that way."

While some knowledge of methods is obviously better than none, dealers warn that a novice can be lulled into a false sense of security in the face of sophisticated forgeries. They advise as the best protection examining original pieces and a few bad ones first hand, talking to knowledgeable dealers and studying examples in books and museums.

Finally, of course, everyone can't possess genuine antiques in perfect condition. Even Clement Conger, when buying pieces for display in the White House and the State Department, says, "If a repair is out of sight in the drawer or back, I couldn't care less.

"Minor repairs don't really concern the average collector," he adds. "A person with limited funds may only be able to buy a piece with a replaced foot, for instance. The important thing, however, is that he knows exactly what he's getting."