This is where the technological methods come to play. The use of neutron autoradiography can detect pigments that may or may not have originated at a specific time period (Fleming, 1980)
A person like Martin Kemp has the background expertise in detecting Leonardo da Vinci forgeries and authentic works. In fact, his expert opinion "can help [a] painting become part of [the] world's cultural heritage" or can "cause [a] painting to be tossed into the trash" (Grann, 2010)
Kemp puts in a mixture of brushstroke analysis, composition, iconography, and pigment evaluation in order to determine the authenticity of the works he is asked to examine (Grann, 2010). Bir? himself has acknowledged the use of DNA and fingerprint analysis to pull physical evidence from paintings (Spinney, 2009)
A forged van Gogh or a forged Rembrandt is a work that supposedly comes from either artist, though the notion is untrue. Forgery is "the intentional misrepresentation of a (supposed) artifact, work of art or not, as far as its issuing from a specified source is concerned" (Wreen, 1983)