When someone finally decided to have the diaries analyzed, they found that the books weren't just fake -- they were primary-school-art-project fake. The forgeries were written in modern books with modern ink and dipped in tea in order to pass as antiques. Furthermore, the text didn't match Hitler's handwriting. "Hitler" even got the color of his own uniform wrong.
The Nazis were famously colorblind.
The journalist who had invested in the diaries, Gerd Heidemann, was such a sucker for Nazi memorabilia that he took the diaries at face value without, you know, running them past a historian or something. The guy he was dealing with turned out to be Konrad Kujau, a notorious forger who made his livelihood by creating fake Nazi relics and selling them to collectors for profit. Kujau did four years in prison for the stunt, but after his release, he was so famous that his forgeries sold better if he put his name on them. In fact, other forgers began to make money by forging Kujau's forgeries.
"You always hear about people wanting to be the next Hitler. Well, this is the next best thing."