The rapid development of biodegradable polymers has intensified the need for reliable, standardized methodologies that can accurately assess their environmental fate. However, biodegradability is not an intrinsic property of materials, but rather the result of complex interactions between polymer chemistry, physicochemical characteristics, environmental conditions, and microbial activity. Consequently, biodegradation performance determined under one set of conditions cannot be directly extrapolated to other environments, highlighting the importance of selecting appropriate testing methodologies. This review provides a comprehensive and critical evaluation of the main international standards used to assess polymer biodegradability, including ISO, ASTM, EN, and OECD methods applicable to soil, industrial composting, freshwater, and marine environments. It discusses the fundamental mechanisms of polymer biodegradation, together with the major factors governing degradation kinetics, and comparatively analyzes the biodegradation behavior of polyesters, polysaccharides, polyamides, and polyesteramides across different environmental compartments. Unlike previous reviews, this work critically compares the applicability, strengths, limitations, and biodegradation endpoints of standardized methodologies, identifies current methodological gaps, and proposes a practical decision-making framework for selecting appropriate tests according to the intended end-of-life scenario of polymeric materials. Finally, it highlights the need for more harmonized and environmentally relevant testing approaches to better distinguish compostability from true environmental biodegradability.