Among the reliable online test series for UPSC Prelims, a few are widely recommended. Notable options include Testbook, BYJU'S, and InsightsIAS. These platforms offer comprehensive and updated question banks that reflect the examination pattern. They also provide detailed performance analytics and feedback, which help candidates identify strengths and areas for improvement. Engaging in multiple test series is encouraged to enhance preparation and build confidence leading up to the actual exam, thereby improving chances of success.