This is a good question. There are many allusions in the book that foreshadow what is to come. This becomes very apparent when you reread the book. I will give you one of the most poignant allusions.
The imagery of fire starts early as the Jews are on cattle trains. There is one woman who screams, "fire." Here is what the text says:
On the third night, as we were sleeping, some of us sitting, huddled against each other, some of us standing, a piercing cry broke the silence: "Fire! I see a fire! I see a fire!" There was a moment of panic. Who had screamed? It was Mrs. Schächter.
As the train continues, the people see chimneys. It all seems harmless from one point of view, but there is an ominous feeling. As the story progresses, they will come to the awful realization of what these chimneys are used for.
And as the train stopped, this time we saw flames rising from a tall chimney into a black sky.