The German bluff was something like that the buildings were for incineration/cremation to reduce the health risks in the camps (where as the mechanism of that death was obscured).
I recall seeing a documentary on this once (on PBS or one of the cable channels).
We may be encountering this same tactic in Iraq today (there are some on the left who deny that the structures we've run across were prisons or torture chambers).
The proof comes in eye witness accounts (and records) and measuring trace amounts of toxins.