As long as there have been works of art that have gained popularity, there have also been countless forgeries. While it takes a keen eye and an immense knowledge of different artists and their styles to know which are real and which are fake, after 1945 there became a foolproof way to spot fakes!
The reason for this is because after 1945, works of art will have traces of the isotopes caesium-137 and strontium-90. Only artwork after 1945 have these traces because they are not found naturally and are only released after a nuclear blast.
There have been over 2000 nuclear tests since the first blast in 1945, which released huge byproducts of these isotopes. When art forgeries arose that were extremely difficult to identify as authentic, scientists learned that they could do a simple test to see whether they were actually produced before 1945 or not, making it the only way we can know for a certainty which are real and which are fake!