Malcolm Gladwell's bestselling book Blink begins with a story about a seemingly ancient marble statue. When several experts in Greek sculpture evaluated it, each pronounced the artifact was a fake. Without a shred of scientific evidence, these experts simply looked over the object and saw that it couldn't possibly have been created during the time period its finders claimed. These experts couldn't, in most cases, explain their beliefs. They just knew.
The experts were able to do this because they had each spent thousands of hours sharpening their instincts through research and practice. They had studied their craft so much that they spotted the fraud almost instantly, though they were often unable to articulate what gave away the object as a fake.