There are two ways to gauge the significance of passages in Wuthering Heights. One is by examining the reception of the novel, the other by judging independently.
For the reception approach, you examine the history of literary criticism, and see which passages are quoted most often by other critics. This method presumes that the significance of a passage has to do with how influential it is. For a university level paper, you could even do a diachronic study of whether the group of passages considered significant have changed over 150 years of criticism, and show how that reflects changes in literary critical trends.
The second approach is based on close reading. It works best if you complete the entire novel and then go back to the beginning to select passages. One type of significant passage is one that occurs early in a novel but foreshadows later events. Another important group of passages are ones revealing the character of Mrs. Dean. Since she is our guide to the events, it's important to look at the passages that reveal her character.