Essays in the UPSC Mains are evaluated on criteria such as clarity of thought, coherence, relevance to the topic, and originality. The structure of the essay plays a significant role, with a strong introduction, organized body, and impactful conclusion being essential. Marks are also awarded for the effective use of examples and data to support arguments. Moreover, the ability to present diverse perspectives while maintaining a balanced tone is crucial. Ultimately, writing style and language proficiency contribute significantly to the overall evaluation of the essay.