Well, usually the main idea in fiction is related to the central conflict. Take 'The Lord of the Rings'. The main idea is centered around the battle between good and evil, represented by Frodo's quest to destroy the One Ring. You can also look at the characters' relationships. How they interact and change in relation to each other can reveal the main idea. For instance, the friendship between Sam and Frodo is crucial in showing loyalty and the power of friendship in the face of great evil.
One way to identify the main idea in fiction is through the repeated motifs or symbols. In 'The Great Gatsby', the green light at the end of Daisy's dock is a symbol that ties into the main idea of Gatsby's unfulfilled dreams and the illusory nature of the American Dream. Also, the overall tone of the story gives clues. If it's a somber tone, like in many dystopian fictions, it can point to the main idea of a bleak future or a flawed society.
The main idea in fiction can often be found by looking at the protagonist's main goal or struggle. For example, in 'To Kill a Mockingbird', Scout's journey of understanding the unjust world around her is a key part of the main idea. It's also about the themes of racism and moral growth that are woven through the story.