They'd had an actual sample of Hitler's handwriting, and all three had confirmed the striking similarities. They ought to have looked into Kujau's background. As a child he'd sold the forged autographs of famous politicians for pocket change. Later he manufactured so-called Nazi mementos, including an introduction to a sequel to Mein Kampf and poems by Adolph Hitler. Forgery detection is a skill often used for document examination.
Document forgery is the fraudulent reproduction of someone else's writing or signature.
Tracing over someone's words can generally be detected because it is difficult to follow the outline precisely. There may also be unnatural pen lifts or an unnatural evenness to the flow of writing. Freehand forgery, such as that which Kujau used, is much harder to detect because it has a smooth flow. However, the forger's own traits are bound to affect the writing and an experienced expert can point them out. When the forgery of the Hitler diaries was exposed, Kujau ran, but was arrested and tried in Hamburg in 1984.