How do we know the Nazis used gas chambers to kill Jews en masse? The same way we prove the Holocaust in general--a convergence of evidence from various sources:
1. Written documents--Orders for Zyklon-B gas, blueprints and orders for building materials for the crematoria, etc.
2. Eyewitness testimony--Sonderkommando diaries, confessions of guards and commandants, etc.
3. Photographs--Not only of the camps, but especially interesting are the secret photos taken of the burning of bodies that were smuggled out of Auschwitz.
4. The camps themselves--forensic tests have now been conducted demonstrating the homicidal use of both the gas chambers and the crematoria for the express purpose of exterminating large numbers of prisoners (Pressac, 1989; Pressac and Van Pelt, 1994).
5. Negative evidence--we have documentation of the numbers of prisoners shipped to the various camps, the numbers that were transferred, and the number liberated. The difference between the latter with the former two figures gives an approximation of the numbers who died or were killed (see Hilberg, 1961).
Proving the Gas Chambers & Crematoria