Read the novel. The process of identifying symbolism in "The House of the Spirits" or any work of literature first presumes you have read the text. Deriving symbolism comes from extracting parts of the whole novel. For this book, pay attention to the characteristics of the Truebas, their relative political and socioeconomic status, and how those affect their actions.
Identify recurring motifs. Symbols often obtain their significance through repetition in order to gain the attention of the reader. In "The House of the Spirits," notice which objects recur during the lives of multiple generations of the family.
Develop hypotheses for the significance of the recurring themes you have identified. A failure to find any significance will help eliminate any features of the book that are repetitive for reasons besides potential symbolism.
Identify "big" themes in the book. These might include objects or events with extensive or unusual description. Allende and other writers reflect their larger values through both plot and symbolism. A statement made in the book--for instance, the portrayal of the Chilean rich and the Chilean poor--may appear in other symbolic ways.