I know that British, Russian, Polish, and American politicians were at least tangentially aware of the existence of these camps, and leadership at the highest level also had an idea of it (if not more). But what would the average non-general (enlisted, non-com's, officers) have known about the Nazi extermination program at the time of the first liberations? Were the soldiers that liberated these camps privy to any rumors, scuttlebutt, or more concerning the labor, concentration, or death camps? Do we have any accounts from these soldiers that shed light on how much they knew prior to liberating these camps?
I would note that the politicians were quite aware of the camps and what was occurring there. The Polish Government-in-Exile issued a detailed report and there were others who had described what was occurring. I answered a related question here which might help in understanding who knew what and when. Since a report had been issued, it is simply up in the air as to which soldiers knew about that proclamation.
Here is a 1939 newspaper article written by a man who escaped from a German concentration camp.