Article 19 of the Nuremberg Charter specified that:
"The Tribunal shall not be bound by technical rules of evidence... and shall admit any evidence which it deems to have probative value."
Article 21of the Nuremberg Charter stipulated:
The Tribunal shall not require proof of facts of common knowledge but shall take judicial notice thereof. It shall also take judicial notice of official governmental documents and reports of the United [Allied] Nations, including acts and documents of the committees set up in the various allied countries for the investigation of war crimes, and the records and findings of military and other Tribunals of any of the United [Allied] Nations.
Given this acceptance of 'evidence' where no proof was ever established, it is clear there were few options for anyone accused, as what they were accused of was already accepted as fact.
Any 'document' or accusation presented by any of the Allied Nations was accepted as fact, without proof.
Also, one must remember that in many countries today it is illegal to question the show trials of Nuremberg. They are trying to hide the tracks.