How do you spot a genuine rare Eames rocker? Check out the shock absorbers. By John Windsor
Take a 1970 Eames rocking chair worth pounds 600. Replace the fibreglass shell seat with a 1952 version and, hey presto! - you have what looks like a rare 1952 Eames rocker worth pounds 1,000.

Except that Christie's South Kensington's modern design specialist, Simon Andrews, has wised up to the fraud. He has found that the rubber shock mounts joining the two incompatible components never quite fit - and that there are sometimes tell-tale scratches left by a screwdriver.

Knowing your shock mounts - a nifty device borrowed from the car industry to give flexibility without upholstery - is all part of the trainspotter- like obsession that collecting furniture designed by the brilliant American husband and wife team Charles and Ray Eames has become. Early models are revered as design icons and rare prototypes are fetching up to pounds 7,500 at American auctions.

The modern look in furniture - minimalistic moulded plywood or metal dining chairs, sumptuously upholstered leather recliners with head-rests and separate Ottomans - is an Eames creation, instantly recognisable, inspired by the need to combine comfort and economy in the post-War world. He trained as an architect and taught industrial design at the Cranbrook Academy in Detroit. She was a painter. In the Fifties, their Los Angeles home, with the tongue-in-cheek name Case Study House, was an open-plan showcase for their innovative furniture.

Now is the time to gen-up on Eames furniture, before prices spiral out of reach. Vintage items can still be bought for less than the ever-popular ones still bobbing off the production line; there is an Eames retrospective at the Design Museum, on London's South Bank, until 4 January, and the first auction dedicated to Eames furniture, at Bonhams earlier this month - which included scarce American-made chairs seldom seen over here - has done much to consolidate prices in a market that has become more mature, discriminating - even pernickety - over the past four years.

Who collects such things? Architects, designers and intensely houseproud people who want to seat their guests on landmarks of design and are capable of holding forth on the evolution of the enclosed metal disc in Eames rubber shock mounts, should the conversation flag.

If you want to avoid being lumbered with later designs in the mistaken belief that they are early ones, you will need to buy fat standard textbooks, with their Meccano-like diagrams of components, and you will also need to get your eye in.

Look at the rare prototype DCM (dining chair, metal) of 1945-6, pictured here. If you saw it discarded in a builder's skip, would you bother to fish it out? The experienced eye sees at a glance that the veneer is rosewood - now an endangered species, no longer used - and that it has acquired a patina. So, for a start, it has age. But it also has a plywood base that is compound moulded - that is, it is bent in three dimensions, not just two, a comfort-giving innovation pioneered by the Eameses.

Further investigation with the help of a textbook reveals that the legs are zinc-coated solid rods, not the later chromed tubular steel, that the plastic feet are the first of three known versions and that the metal discs of the shock mounts - yes, it's those rubber shock mounts again - are exposed, not enclosed as in later, mass-produced models. Final clue that this is a valuable prototype: a hand-welded join in the metal T-support of the back-rest.

In fact, only six such prototypes were made, and this one has provenance that adds to its value - the Eameses made a present of it to their neighbours in California, the celebrated stage and screen performers Alexander and Doris Knox, in 1951.

It has never been thrown into a skip. Instead, it sold for a hefty pounds 4,600 at Bonhams this month, within its pre-sale estimate of pounds 4,000-pounds 6,000. At first glance it does look like an ordinary kitchen chair - but that is because its design has passed into common currency. It is no ordinary design.

Seasoned collectors check over not only the nuts and bolts but also the manufacturer's labels. They must be from the right manufacturer, such as "Evans Products Co", which worked closely with the Eameses and used four different labels in different years. They must also to be intact, rather like the dust covers of modern first-edition books. A label with a bit missing will actually lower the price. As Christie's South Kensington's Simon Andrews says: "All these little details are great fun - but they can also drive you mad".

To prove authenticity, he advises cross-referencing design details that have known dates. Here is a worked example: a rocking chair designed in 1948-50, made by Zenith Plastics, that sold for pounds 977 in Mr Andrews' October sale, topping its pounds 700-pounds 900 estimate. He dated it to the first production batch of the late Forties or early Fifties because of its grey colour.

Also, it has edges of reinforced, embedded cord, used to strengthen the moulded fibreglass - a feature abandoned in 1954-55 when harder fibreglass was developed. And the shock mounts? Authentic, pre-1955 models.

At auction you can still buy Eames DCMs, DCWs, LCMs and LCWs (dining and lounge chairs in metal and wood) of the Fifties and Sixties for pounds 200- pounds 300.

In Christies South Kensington's October auction, an Eames lounge chair with Ottoman - the 670 and 671 models of 1956 - by another celebrated maker, Herman Miller, fetched pounds 1,840. These days, they seldom sell for less than pounds 1,800 at auction. Two years ago, you might have picked one up for pounds 1,500, four years ago for pounds 1,200. A superb 1956 specimen with pale, beautifully figured rosewood veneer, made pounds 2,990 in the Bonhams sale. But they need to be in absolutely tip-top condition. Leather may be crinkled, but definitely not split. Damage reduces value drastically. And beware 670 models with backs that have worked loose: that, dare one say it, is a design fault.

Simon Alderson, founder of the London modern furniture dealers twentytwentyone sells Eames recliners in excellent condition for pounds 2,000-pounds 3,500. Vitra, which has succeeded Herman Miller as the licensed manufacturer of Eames furniture, will sell you a brand-new version of the 670 and 671 recliner with Ottoman for pounds 3,167.80, including VAT - in cherry, not rosewood. Second hand, in Mr Alderson's shop, they sell for around pounds 1,500 - which should encourage you to start shopping around for a vintage model.

Recommended textbooks: the bible is Eames Design by John and Marilyn Newhart and Ray Eames, published by Thames and Hudson (1989). Also: Charles and Ray Eames: Designers of the 20th Century by Pat Kirkham, by MIT Press (1995) and The Work of Charles and Ray Eames: A Legacy of Invention, Vitra Design Museum (1997).

The exhibition: "The Work of Charles and Ray Eames" is at the Design Museum, Shad Thames, South Bank (by Tower Bridge) London SE1 (0171-403 6933) until 4 January. Entry pounds 5.25, concessions pounds 4 . Bonhams (0171-393 3984): next Design sale, 24 February, 6pm. Christie's South Kensington (0171-581 7611): next Modern Design sale 17 March, 2pm. Dealers: twentytwentyone, 274 Upper Street, London N1 (0171-288 1996), Tom Tom, 42 New Compton Street, London WC2 (0171-240 7909). Licensee/retailer: Vitra, 13 Grosvenor Street, London W1 (0171-408 1122).