A painting that went for $60 in 1958 sold for a record $450 million at a Christie’s auction in November 2017, the highest price ever paid for a painting. That’s because the piece—known as Salvator Mundi, or “Savior of the World”—was identified as a work by Leonardo da Vinci in 2011. Painted around the same time as the Mona Lisa, it is one of fewer than 20 known paintings by the Renaissance master.

Why did it take this long for art historians to figure out the luminous early 1500s work was by da Vinci? Well, from the 15th century onward, many artists created Salvator Mundi imagery, usually showing Christ raising his right hand and cradling the world in his left. Art historians in the 20th century knew that da Vinci had created his own version of Salvator Mundi, and had identified more than 20 copies that other artists had made of his version. But for a long time, no one knew where the original was.

It wasn’t until 2011 that experts revealed that one of the supposed “copies”—specifically, the one that had once sold for $60 (about $510 today)—had been misidentified, and was actually da Vinci’s original work.

Because finding a da Vinci is such a big deal, there’s never a shortage of claims about possible “lost” works of his: Since 2015, scientists at the Louvre have said they might’ve found the “nude Mona Lisa”; a French doctor has claimed he has one of the artist’s lost drawings; and some have questioned whether a recently “rediscovered” da Vinci painting is actually a forgery or a copy.

Art historians and conservators use a range of criteria to evaluate possible da Vincis when they arise, including technology that earlier art experts didn’t have access to. This gives modern researchers a huge advantage over their predecessors, says Rita Albertson, chief conservator at the Worcester Art Museum in Massachusetts, who has also been researching another possible da Vinci, called Miracle of San Donato, for at least a decade.

The painting is a small scene panel from a larger altarpiece that was created by the workshop of artist Andrea del Verrocchio in the late 15th century, according to its commission document. At the time, Leonardo da Vinci worked in that workshop, where he and other artists collaborated on assignments. Using advanced technology to examine the painting’s underdrawing, Albertson says her team has found multiple drawing styles at work, and that they believe one of them could be da Vinci’s.

The “different hands in the underdrawing,” she says, technologically “could not have been detected up till now.” But researchers have also collected other clues that a computer can’t necessarily spot, like da Vinci’s scientifically-based observational skills. These signs include da Vinci’s unique way of observing how light falls, and his ability to make two-dimensional objects appear three-dimensional.

“He was ravenously hungry to understand how things worked and how things grew,” Albertson says. For example, in Miracle of San Donato, she says the trees seem to be “growing up from out of the ground. They’re very organic looking as opposed to sort of cookie-cutter trees which you see in other works from this workshop.”

Albertson explains that identifying da Vinci’s earlier work is different from identifying his later work because there are fewer examples for comparison. In addition, Albertson thinks researchers shouldn’t assume that da Vinci’s earliest pieces will be as mature as the ones we know him for.

“We’re looking for signs of brilliance,” she says. “We’re not looking for perfection.”

Sometimes it can take half a millennia for true brilliance to be identified, as in the case of the rediscovered masterpiece of_Salvator Mundi._