Art detectives Philip Mould and Bendor Grosvenor are expert in unearthing the secret, lost works of the greatest painters, but how do they do it?
Do you know who painted the unassuming picture hanging in your living room or the one abandoned in the attic? If the answer is no, you're in good company.
Astonishingly, there are over 17,500 "unattributed" oil paintings - or by an "unknown" artist - in UK public art collections alone.
This huge Aladdin's cave of "orphaned" pictures is prime hunting ground for Fake or Fortune's Philip Mould and Bendor Grosvenor, together with co-presenter Fiona Bruce.
In the final episode of the series they investigate a number of unattributed works spotted on the BBC Your Paintings website, in the hope they are actually by the hand of Thomas Gainsborough, one of Britain's greatest ever artists.
These potential Gainsboroughs could be highly valuable paintings that have been, until now, off the artistic radar or tantalisingly known as "sleepers". If this is the case, how many other works by great masters could be secretly hiding in our national art collection?
And if they are by Gainsborough, how could they have slid from being highly prized possessions and status symbols to the wilderness of a museum storeroom filed under "artist unknown"?
Unfortunately, many paintings lose their association with an artist years before they are gifted to a museum, in the same way other family possessions might be overlooked or neglected over generations.
But it's condition that Philip Mould thinks is the most common reason they are lost from view or wrongly catalogued.
"The quality and the idiosyncrasies of a painting can get lost in time through the accumulation of dirt; varnishes turning yellow and brown; and through over-paint [during restoration] which can cover the defining brush strokes that will attach a work of art to an artist."
Other complicating impairments can include: canvases hacked down to fit into another frame; over-zealous or amateur cleaning which can strip away colour; smoke damage from tobacco or fires; and details painted out or in, including the re-dressing of nudes or the brightening of sunrises.
But what about the signature? Whilst many artists understandably recoil from signing the front of their canvases (it detracts from the composition and the flow of the work), surely a great painter would sign the back for posterity?
"The concept of signing your work - the notion of artist's celebrity - is a more recent thing," Philip says. "Artists of the 16th, 17th and 18th Centuries generally didn't do it.
"There wasn't an imperative to bang on a signature... but what is a signature? Anyone can put one on a picture."
This cocktail of camouflage could render a painting unidentifiable to many experienced art historians, but not Bendor and Philip. "You develop an eye," says the latter.
"You learn to understand pictures that have been damaged and may not look like a work of great quality but are - they are just compromised.
"One always feels that tingling if one feels that something is right!"
So in the absence of any immediate documentation, a lack of signature and a picture that's unrecognisable, how do they go about hanging a certain attribution on the painting?
"It's all about building a case so you can then say 'this is the guy - or lady - whodunit'," says Bendor.
Bendor, Philip and Fiona approach an orphaned canvas like an unsolved crime, using a combination of art-world expertise, intuition and forensic analysis.
They have an enviable clear-up rate in spotting lost works by great artists. In the last series they successfully proved one of Philip's own purchases was actually by Anthony van Dyck, the genius portraitist.
There is also a lot of good old-fashioned detective work. Philip and Fiona grill numerous art experts and Bendor, as head of research, is often seen trawling through the archives of galleries and museums researching the paintings' histories and provenances. "I think part of me is a frustrated police detective!" he laughs.
"One of the reasons we can make more discoveries than ever before is because we have digital images in a way we just didn't have 25 years ago," says Philip. "The Your Paintings website has given us a whole new playground.
"For the first time we have a colour [online] inventory of Britain's works of art. It's a bit like going through a shopping list… it gives you leads and opens up little windows."
And, of course, the real painting itself is interrogated. Besides unique brushstrokes, every great artist leaves stylistic fingerprints across a picture.
This could be exquisitely handled drapery, a unique softness of expression or a masterful depiction of light. A knowing eye will locate and identify these tell-tale traits quickly.
"It basically comes down to what we call 'connoisseurship', or the ability to recognise an artist's style," says Bendor. "You can recognise the way he paints just like you can recognise the handwriting of a friend.
"But although you might be completely satisfied and going 'A-ha! That's by Van Dyck!', you still have to convince everyone else," he continues. "And nowadays, increasingly, science plays a role."
The battery of forensic analysis that a canvas can undergo includes infrared radiation testing, which penetrates beneath the varnish and paint to reveal what hides beneath. This could be sketching or preparation typical of the artist.
Some of the tests can also be useful in combating the dark side of the art world - fakes. The lasers and scans which identify the chemicals used in paint can help date a picture, often scuppering the claims to originality of an 'older' painting.
However, as Bendor points out, the science can only reveal what a picture isn't, rather than what it is - and it can always be misread by humans.
"All the time I come across examples of scientific analysis being hopelessly misinterpreted. People look at an X-ray of a painting and imagine they can see a signature and they are just being wildly optimistic."
But science is moving at a terrific pace. In 10 years' time, could we be seeing the Fake or Fortune team unearthing some definitive DNA evidence?
Bendor is more sceptical than Philip, musing that if they did manage to find a genuine hair from, say, Van Gogh, on a painting, the most it can really confirm was that he was in the general vicinity.
He believes the future of research lies more in object recognition by computers and databases through which paintings can be compared globally. This, he predicts, will also make people like him redundant.
Once the evidence is assembled, the respected authorities on the artist have to be convinced this is truly a rediscovered work and validate it. This does not always go smoothly.
In one recent episode about the modernist painter Marc Chagall, the painting in question was ruled a fake.
The team were unsure about its authenticity themselves but were still shocked when the Chagall Committee ordered that it also be destroyed.
Although not every endeavour ends in celebration, the trio quite clearly enjoy the thrill of the chase. But finding a sleeper and proving its thoroughbred obviously appeals on a deeper level.
Bendor says it is satisfying to right the wrong of a great work which has been "unfairly maligned".
Philip agrees. "It's like they are posthumously keeping an eye on you in return... as if they are tapping you on the shoulder from beyond the grave.
"I just love Gainsborough... and he's influenced my sensibilities to art. So I feel as if I owe him one really."
For every known masterpiece, they believe another may be waiting to be discovered. Could the next one be by Thomas Gainsborough?
A fourth series of Fake or Fortune is being planned. If you think you might have uncovered a sleeper but need some help taking the next step, please contact the production team.