UPSC examiners evaluate answer sheets in the Main Examination through a systematic process. They focus on various criteria, including relevance to the question, clarity of expression, structuring of answers, and the depth of knowledge displayed. The evaluation is aimed at discerning how well candidates articulate their understanding of the subject matter, adherence to the question format, and critical thinking. Each answer sheet is subjected to multiple assessments by different examiners to ensure fairness and accuracy in marking, thereby upholding the integrity of the examination process.