Aspirants can identify reputable study materials for UPSC subjects by consulting successful candidates, educators, and online forums dedicated to UPSC preparation. It's important to look for materials that are aligned with the UPSC syllabus and are updated regularly to include current affairs. Popular sources include NCERT textbooks, standard reference books, and curated online courses. Additionally, reviews and recommendations from trustworthy coaching institutes can offer insights into effective materials. Candidates should prioritize comprehensive and clear resources that cover various subjects effectively while focusing on quality over quantity.