How do you find a central argument in a non-fiction novel?
This question is somewhat puzzling as there is actually no such thing as a non-fiction novel. A novel is defined as a long work of imaginative literature written in prose. Thus all novels are fiction, even if they include some realistic or historical elements.
A work of nonfiction is not a novel. Instead, it is based on facts or events that really happened. While some genres of non-fiction, such as biography or history, may have narrative components, because they are real rather than fictional, they are not novels.
Thus a work can be either nonfiction or a novel but not both.
Some works have arguments and some do not. Novels and biographies are both narratives which tell stories rather than works which argue a specific point. Sometimes writers will tell stories to create sympathy for a point of view. For example, Black Beauty is a poignant tale evoking sympathy for mistreated horses, but most narratives are stories rather than arguments.
For works of non-fiction, some have arguments and some don't. For example, a physics textbook or a field guide to birds or a book on how to use a computer just organizes information rather than arguing something.
Argumentative or persuasive books are written to make a point or to argue something or to urge people to act a certain way. They usually set out their main arguments in the introduction and recapitulate them in their conclusions. You can identify the main argument of a book by looking for the central point that all the other evidence in the book supports.