Well I'm sure other people have different ways to do it, but you can either decide on what you think belonging is and look for that whilst you read the book, or read the book and form what you think it is after.
I said something like "Belonging is a state in which we find a person, activity or place that can make us feel secure and accepted". Making your own isn't too hard.
Some ideas of belonging from the book that I can remember is the closeness of the father son bond. The related text I chose was the film "A Beautiful Mind", and if you have seen it, it can link to Romulus quite well. In both films, we see a sense of belonging blossom between two characters, and subsequent to the mental illness of the protagonists, and their hospitalizations, the relationships become stressed. You could examine how illness inhibits our ability to belong.
You could view their descent into mental illness as a result of their desire to belong. For Romulus, his becomes destabilized after receiving the rejection letter from Lydia, whom he hoped would come and be his wife. For John Nash, in A beautiful mind, his devotion to his mathematical work that he obsesses over and finds a sense of security in, leads to his mental breakdown as well.
Ironically, both of their desires to belong become excessive and end up preventing it.
There are other texts you could maybe fit those ideas into, but there are others that are more general. You could examine the idea of making sacrifices to belong. Romulus continuously makes financial and personal sacrifices for the benefit of others, and mainly his son. An example of this is Romulus' choice to clean the toilets at the Baringhup camp so he may supervise Raimond.