How to spot an art fake This "Picasso" failed to dupe people at auction, selling for less than £100 Some believed they might have works by Monet, Renoir and Turner. There were a few who thought they had paintings by prominent, but lesser known modernist Russian, German and English painters - the type that only make modest tens or hundreds of thousands at auction rather than millions. Looking at them together, as we did a month later to narrow down choices, traits began to emerge like fingerprints at a crime scene. First came the back story. Or lack of it. Most of these suspect works had been bought from small-time art and antique dealers, internet sites or minor auctions. If the owner had been given provenance that went back earlier - something that is usually crucial for the authentication of a 20th-century art work - it sounded about as plausible as the dog ate my homework. One innocent purchaser had been told by the seller of a manifestly fake Salvador Dalí that it had come from a "down-at-heel artist" who had approached the previous owner's uncle while he was having supper in a Madrid café in the Twenties. On his return to London, and after looking at photos, the uncle then "realised" that the artist was the spitting image of Dalí himself. It did the trick. The purchaser had willingly paid £1,700 for a trapper worth little more than a plate of paella. That is not to say, however, that they cannot draw more financial blood in the right conditions. Some years ago I watched incredulously as a "mid 20th-century Russian" abstract picture was hammered down for £13,000 in a regional sale, a folly that required not one but two eager buyers to force its price up from the low hundreds. It is also possible that trappers that have started life at £100 or so, once they have changed, can move higher up the art-world food chain after they have achieved more form and provenance. Trapper makers know their limitations. Rather than risk creating new compositions they will generally copy or bastardise less known examples of the artist in question, culled from a growing multiplicity of options online, in books and from auction catalogues. They also favour impressionistic, abstract, or more simplified artistic styles that do not require the high finish and more measurable technical accomplishment of traditional old masters. A dentist from London who collected antique watercolours and should have known better, paid £1,800 in a sale in the West Country for a mouth-gaping visage on canvas that purported to be by Chinese painter Yue Minjun. Its easily replicable, chintzy style, combined with the scarcity of comparable works in Britain to judge it by, made it a felling ruse. Not that who the artist might be is ever a legal issue. For the sweet twist suffered by almost all trapper buyers is the bill of sale, verbal or otherwise. Terms such as "French School," "20th-century British School" or "Manner of... [the artist in question]"proliferate. Unlike a major fake, no firm attribution is ever given by a trapper merchant. Thus the seller need not be guilty of what Winston Churchill - another highly valuable artist who I have seen "trappered" - neatly termed "terminological inexactitude." The buyer's misplaced optimism effectively consummates the deal. The visual heist is not just confined to the image. Creative energy is applied by trapper-makers to other components of the art work such as the frame, backing-board and nails (often artificially rusted). A Picasso trapper that came up in a sale in southern England compensated for its lack of visual plausibility by what lay on the reverse. Splashes of paint covered an artificially tea-stained label, upon which was inscribed, in misaligned, Fifties Remington typewriter script: "EXHIBITION - PARIS." So why has this form of fakery mushroomed in recent years? It is partly, of course, human nature - a heightened dream of discovery, fuelled by a buoyant art market, television, books and media. Fantasy, a misplaced hunch, and the fact that the financial outlay may only be in the hundreds of pounds appears enough to fuel a thriving home market. But the depressing aspect to all of this is that artistic talent and energy is funnelled into duplicity. A group of competent painters is happy to pursue a career in the twilight zone, falsifying the work of others rather than the worthier, and creatively more risky task of being themselves. If beauty is truth, their art is unmitigated ugliness. Tips for spotting a fake Check the age of the mount and the frame. Works by prominent 20th-century or earlier artists have normally been framed or mounted for generations. Trapper-makers also reuse and adapt normally cheap, inappropriate frames. Never base a decision on a signature. They are not like number plates and the easiest thing to add or fake. Trapper-makers also sometimes add deliberately obscured signatures that might intimate the artist's name without being properly readable. If a painting is a fraction of the cost of what is obviously the real thing, it normally isn't. Ask the seller, if you can, to guarantee in writing that a picture of some apparent age is not modern. He might run a mile, but that in itself is indicative. If the image looks familiar, google the artist. The likelihood is that it will be based, or reliant upon on a painting that has been reproduced or illustrated somewhere findable. Look for heavy-handed signs of provenance-adding on the reverse such as famous names, collections or exhibitions that trapper-makers have a tendency to overplay. If you can, take it on approval, or take a decent photo, and show a specialist dealer or auctioneer if you need advice. Most will offer it on a friendly basis if properly approached and not quoted (unless willing to be so). Series three of 'Fake or Fortune?" presented by Philip Mould and Fiona Bruce is on Sundays, BBC One, at 6pm