I visited the Imperial War Museum holocaust exhibition late last year, and this has some information about the reporting of concentration camps. The existence of concentration camps was reported in Britain and the US prior to 1944. However some of these reports were toned down because they were so bad they sounded like propaganda.

Whatever the information available to the allied soldiers, it probably couldn't prepare them for the reality of the camps. Conditions would have deteriorated as the situation deteriorated for the German regime. In at least one camp the guards disappeared some days before the allies arrived, leaving the starving inmates. I also don't know how much information was circulated about the industrial extermination of people. Again, I think this was viewed as hard to believe.