The reader realizes that the book is science fiction when animals don’t exist.
There are hints that Jonas’s world is different from ours from the very first page. First of all, he is frightened by a plan flying overhead and nervous about a Ceremony of Twelve. The word “Pilots” is capitalized, as are other jobs. A voice comes through a loudspeaker and tells everyone what to do, and everyone does it.
While these and other hints make the reader wonder where the story happens, the fact that this world is different in a more impossible way is revealed when they do not know what animals mean.
"That's right," Lily said, laughing too. "Like animals." Neither child knew what the word meant, exactly, but it was often used to describe someone uneducated or clumsy, someone who didn't fit in. (Ch. 1, p. 5)
Although we later learn that Jonas has visions and all of the people are genetically engineered, while the weather is controlled, the animals are the first sign that something is off. After all, an elephant does not exist.
The slow buildup of differences adds to the suspense of the story, helping the reader see that this perfect world is actually far different from our own, even though many things are the same.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book) (p. 5). Houghton Mifflin Harcourt. Kindle Edition.