The UPSC assesses candidates' performance through a rigorous evaluation system that includes the Preliminary and Main Examinations, as well as an interview stage. Performance in the Prelims determines eligibility for the Mains, while the mains examination evaluates subject knowledge, and the interview assesses personality, communication skills, and ethical judgment. This multi-faceted approach ensures a comprehensive evaluation of candidates, leading to the selection of well-rounded individuals for civil service roles.