How can I determine the purpose of this novel?
Since a novel is essentially fictional and the author does not address us in propria persona, it is difficult to determine intentionality from the text. One cannot assume that even the novel's narrator speaks for the author, as Victorian narrators tend to function as quasi-fictive characters.
The two main ways to determine intentionality are via certain theories of genre and via biographical research.
For biographical research, it is possible to examine biographies of Hardy, including his letters, in order to determine what he considered the novel's purpose. One consistent comment he makes is that he thinks of the novel as a way to earn money in order to leave him free to devote his time to poetry.
You can also thoink about the social function of the novel, although that is not precisely the same as authorial purpose. The purposes of readers and publishers may also be distinct from authorial purpose.