The electrocutions at Belzec (and electrocutions at Treblinka) feature in Nuremberg document USSR-93. Although you won't find this particular document reproduced in the IMT, NMT, NCA volumes, an English translation is available in NARA.
General Rudenko said the following about USSR-93 at the Nuremberg trial, February 20, 1946:
I shall now pass to the reading of the fourth section of the official report of the Polish Government dealing with crimes committed by the Hitlerites in occupied Poland. This report has already been presented to the Tribunal as Exhibit Number USSR-93 (Document Number USSR-93) and, according to Article 21 of the Charter, constitutes irrefutable evidence.http://avalon.law.yale.edu/imt/02-20-46.asp
Article 21 is of course:
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 Nations, including the acts and documents of the committees set up in the various allied countries for the investigation of war crimes, and of records and findings of military or other Tribunals of any of the United Nations.http://avalon.law.yale.edu/imt/imtconst.asp#art21
"Irrefutable evidence" (i.e. a report by the Soviet-controlled Poles) that Jews were electrocuted at Belzec and Treblinka was presented during the Nuremberg trial. I can't understand why Holocaust denial even exists.
Unfortunately, the "irrefutable evidence" contained in USSR-93 about Jews being gassed and electrocuted in Treblinka, is completely contradicted by further "irrefutable evidence" in the form of Nuremberg document PS-3311, another "Polish" report which states that Jews were not gassed and electrocuted at Treblinka, but steamed to death.