How do we prove something in history?

Are you referring to Holocaust denaial? Irving is a Holocaust denier; this means exactly what is indicated. He espouses the lie that the Holocaust never happened; yes, there were internment camps where Jewish people, as well as Catholics and political opponents of Hitler were imprisoned, but he claims that these were not death camps. He even comes up with a media-friendly epithet to labor his point - "no holes, no Holocaust" - and to state that there were no holes in the roof of the buildings that were said to be gas chambers, therefore there was no way that gas could have been introduced. The film also shows that Holocaust denial is a tool used by anti-Semites and fascists to promote their own agenda.