Look for words that specifically describe one character's relationship to the other, such as "brother," "friend," "rival," "husband" or "wife."
Identify any intense ups and downs in the relationship between two characters, as this may signal a budding romantic relationship. For instance, if one character really likes another at one moment, and in the next moment thinks that she's being unkind towards him or ignoring him.
Find verbs that describe the feeling of one character toward another, such as "love," "hate," "like" or "admire."