I think that "fair weather" friends show themselves in their everyday behavior. For example, they don't have much interest in you or other people. Instead, they just want to talk about themselves and menial things going on in their world.

Whereas a "true" friend asks questions and listens to your response. They respond to what you say, maybe making a related remark or asking another question in response to what you had to say. When the subject is over, the conversation moves onto things happening in their life.