I’ve seen two different pieces of “evidence” of a “hoax”, and I’m sitting here going “Surely, there are ways to debunk this.”
1.) The aerial photos showing smoke coming from the smoke stacks of crematoriums do not have certain shadows projecting from the smoke stacks
2.) The soil texture in the concentration camps were not deep enough for mass graves
/u/commiespaceinvader's post on combating Holocaust denialism includes much of what you're looking for. I'd also recommend checking out their entire profile!