Art forgeries can be detected several ways, some of which require detailed technology such as X-rays and infrared reflectography. When an authentic painting is placed under an X-ray, “an artist’s revisions or even abandoned compositions on a re-used canvas” can be seen (Steve Schlackman, “These Techniques Can Detect Art Forgery,” Art Law Journal, May 3, 2018, https://alj.artrepreneur.com/techniques-art-forgery/). Another way to determine a forgery is to inspect the paints used and compare them with what was available at the time the piece was claimed to have been made. Some materials weren’t available at certain points in history, while others are no longer used because of the risk they create. This illustrates the practice of determining authenticity by what isn’t there. You can spot the fraud by the omission.