How can I determine the main themes of a novel?
Reading a novel to discover themes is an early stage of learning how to read critically and organize your response in terms of analytic structures rather than simply plot summary and chronology.
As you read through the novel, look for similarities in ideas, situations, imagery, or emotions which recur across multiple characters and places in the plot. For example, in Jane Austen's novels, there is an implicit assumption that gentleman marry among their own class, and a recurring theme of the conflict between gentry as a cultural/social construct and as an economic one. This leads to a second theme of whether social mobility, and the rise of urban fortunes, destabilized the organic unity of class fusing the cultural with the economic.
A typical ideological theme would be whether natural and human law, or nature and civilization, conflict with each other (e.g. in Antigone) or reflect each other (Pope -- But when t'examine every part he came/Nature and Homer were, he found, the same.")