是真还是假？

What would you do if you were told that the masterpiece you bought for £100,000 was actually a fake? That's what happened to British businessman Martin Lang. He thought he was the proud owner of a painting by Marc Chagall, which he bought in 1992. But, recently the Chagall Committee in Paris ruled it wasn't genuine and should be destroyed.

Forgery, the ugly side of the art world, is big business. Last year in Germany, police raided warehouses and art galleries across the country in an operation to smash a ring of forgers specialising in works by Wassily Kandinsky and other avant-garde artists. In Wiesbaden they seized more than 1,000 objects.

And how difficult is it to find out if a painting is authentic? When verifying if an artwork is the real deal, experts take into consideration if it's done in the particular style of the painter it is attributed to; with his or her characteristic brushstrokes, themes and technique. They also analyse if the materials used – canvas, paint and the frame – belong to the particular period in which the painting is supposed to have been produced.

Following the paper trail also helps those who don't want to be duped. Philip Hook, senior director of Impressionist and Modern art at Sotheby’s in London, says it's always good to look at the back of the painting. That's where you may find inscriptions by the artist, labels attached by dealers who've handled the picture and labels from exhibitions in museums – all of which can be checked.

But Hook believes in some cases spotting a counterfeit is difficult. He explains: "The easiest art to copy and get away with it is art done within the past 50 or 100 years because that submits less easily to chemical analysis. For instance, Van Goghs have been proven not to be by Van Gogh through the analysis of a pigment of the paint and it's been revealed that a certain sort of white wasn't invented till 30 years after his death. But the closer you get to present day, the less relevant this becomes."

It remains to be seen if future technology will help those who deceive in the art world - or those who uncover deception. One thing is for sure: if genuine masterpieces have a place in a museum, then forgeries do too - or at least stories about them. Yes, they do: in Vienna, there's a Museum of Art Fakes.