Although Jane Yolen's novel The Devil's Arithmetic is written in third person, it's also clear that it is written in a third person limited point of view, which means the narrative is told by focusing on only one character, which is the main character or protagonist. We can tell by looking at just the first couple of chapters that the main character is Hannah due to the number of times she's mentioned.
Chapter 1 opens with Hannah declaring, "I'm tired of remembering," which is a central theme in the book. After this first reference to Hannah, we see that Hannah's name is the most frequently referred to name in the chapter. Plus, any time the author uses the pronoun "she," the author is referring to Hannah, as can be seen in the following sentence:
Sometimes she wished her mother would yell at her the way Rosemary's mother did, but she knew her mother would only give her one of those slow, low, reasonable lectures that were so annoying.
The pattern continues in subsequent chapters--each chapter begins with a reference to Hannah, and it is through Hannah's eyes that the reader views the story because Hannah does the most talking and the only thinking.
Beyond noticing from whose perspective the reader views the story, the main character can also easily be determined by thinking about which character does any learning or changing. The reason why is because the main character is the protagonist, and it is the protagonist who is faced with and must overcome the story's conflict. In The Devil's Arithmetic, the central conflict is that Hannah takes for granted the value of her family, the value of life, and does not have a true understanding of the horrors survived during the Holocaust. In the plot, Hannah is taken back in time to the Holocaust where she herself is herded into a concentration camp. Hannah undergoes a transformation in the concentration camp as she learns what it truly means to develop deep relationships and how painful it is to truly experience loss. The greatest turning point arises when she sacrifices herself by taking her friend's place in the gas chamber. After that moment, the plot is resolved when Hannah finds herself back in the present of the 1980s and looks at her family in a whole new light.
Hence, since we know it is Hannah who undergoes a transformation in the novel, we also know that only Hannah can possibly be the main character and protagonist.