In Erich Maria Remarque's "All Quiet on the Western Front", Chapter 6 there are two major signs that the Germans are starting to lose the war.
The first of these is the state of the two sides' artillery. The Allies' artillery is increasing in number. By contrast, German artillery is wearing out -- enough so that some shells are landing in the Germans' own trenches.
The second sign is that many of the German troops are young and inexperienced. Because they are not veterans, a much higher percentage of them die.
These signs show that the Germans are losing the war.