In World War II, when did Germany's commanders realize they were going to lose, and that the US was a major reason?

It seems rather obvious in retrospect that Germany would lose a war fought on two fronts against most of the rest of Europe and the US, especially as they had the experience of losing in a similar situation not long before, in WWI. Simply put, Hitler's strategy ( if you could call delusional racist fantasy a strategy) was to make lightening-fast assaults against the opposing countries, quickly collapsing the weak communist sub-Aryan governments that ruled them, colonizing and securing the conquered regions with Aryan settlers. Britain would be forced either to surrender or negotiate, and then the US would be faced with considering invasion of a fortified Europe.

The question then becomes, given this was very unlikely and also insane, when did the German military realize it? Years ago, the dominant theory was a "clean Wehrmacht" : professional German officers are ordered by Hitler to embark on war of conquest. They have doubts, but they follow their orders and are surprised when they keep winning, gaining the Sudetenland, Poland, France...and, initially, face exactly the badly-wielded Russian armies Hitler's racism seemed to predict. But when Stalingrad happens, they are forced to realize that the project is doomed and, worse, that Hitler is not going to negotiate for terms... that for him the racial struggle can have no surrender, that Germans must fight to the very last life. After this point, ( February , 1943) there are officer-led plots to kill him that, unfortunately, do not succeed.
The "just following orders" excuse was a common one heard later at the Nuremberg Trials. It seemed plausible that it was mostly Hitler and a few others who were insane, because with his death ( and the death of Goebbels, et. al) the German Nazi movement collapsed- they didn't try to carry it on. The story of clean Wehrmacht also gave the many surviving German soldiers and officers something to hide behind, in the difficult years after the war when they had to rebuild a destroyed country.

Unfortunately, it does not seem as though it was that simple. Rather than being completely professional, a lot of the Wehrmacht seems to have, at least, gone along with and, at worst, embraced Hitler's racist delusions, assisting the SS and SD with their mass murders, etc. Stalingrad didn't confirm that Hitler's plan was really insane, but the more basic discovery that he was a terrible tactician, first demanding total authority over all troop movements in the Stalingrad battle and then being completely unable to give them orders to do anything. Wolfram Wette perhaps has the most noted book on this, and he does have a lot of data on his side. And it makes a good bit of sense when speaking of the commanders: Hitler was an ideologue who wanted to be surrounded by other Nazis, not detached professionals.

So, although there would be a difference between all the top commanders having grave doubts before Stalingrad and very few commanders having grave doubts, if any, before Stalingrad your answer would still be, essentially, February 1943, after Stalingrad: when there is no question what's going to happen. How much a role the US played in this is a bit more debatable- the US was in the war, supplies were flowing into Britain and the US had landed in Africa. In the long term, the US was going to be a threat, but the defeat at Stalingrad was before the major US victories, happened even before Rommel was defeated in Africa.