Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Moral and Firm
In Number the Stars, there's never any question about what's right and what's wrong. All the Danish characters we meet share a united front: what the Nazis are doing is terrible, and the Danes must do whatever they can to stop it. Annemarie's family doesn't hesitate for a moment. They will help the Rosens no matter how dangerous it might be. There's simply no other option. Annemarie's father sums it up pretty nicely:
"We don't know where [the Germans are taking the Jews] and we don't really know why. They call it 'relocation.' We don't even know what that means. We only know that it is wrong, and it is dangerous, and we must help." (4.68)
Number the Stars very clearly separates the people doing right (the Danes) from the people doing wrong (the Nazis).
In case we didn't get the picture during the story itself, Lowry drives the point home in her Afterword. The author shares the story of a Resistance fighter named Kim Malthe-Bruun, who remained hopeful even up to his execution by the Nazis. She recommends—strongly recommends, we might add—that people today must keep working on Malthe-Bruun's dream to "create an ideal of human decency" (Afterword.19).
Bottom line: both the voice of the narrator and the voice of the author scream strict moral code. And you know what? In this case, we're totally down with it. Human decency sounds pretty good to us.