After WWII, in 1946, Rudolf Hoess, Commandant of Auschwitz, gave testimony at the Nuremburg Trials, and this is part of what he said:
Another improvement we made over Treblinka was that we built our gas chambers to accommodate 2,000 people at one time, whereas at Treblinka their 10 gas chambers only accommodated 200 people each. The way we selected our victims was as follows: we had two SS doctors on duty at Auschwitz to examine the incoming transports of prisoners. The prisoners would be marched by one of the doctors who would make spot decisions as they walked by. Those who were fit for work were sent into the Camp. Others were sent immediately to the extermination plants. Children of tender years were invariably exterminated since by reason of their youth they were unable to work. Still another improvement we made over Treblinka was that at Treblinka the victims almost always knew that they were to be exterminated and at Auschwitz we endeavored to fool the victims into thinking that they were to go through a delousing process. Of course, frequently they realized our true intentions and we sometimes had riots and difficulties due to that fact. Very frequently women would hide their children under the clothes but of course when we found them we would send the children in to be exterminated. We were required to carry out these exterminations in secrecy but of course the foul and nauseating stench from the continuous burning of bodies permeated the entire area and all of the people living in the surrounding communities knew that exterminations were going on at Auschwitz.
The source for the above quote is the Modern History Sourcebook, an internet-based documentary project of Fordham University.
More of Hoess’s Nuremburg testimony here.