Stable and efficient wastewater treatment is increasingly important amid growing water scarcity and complex pollutant loads. Physicochemical parameters (BOD5, COD, nutrient concentrations) remain the primary monitoring tool at treatment plants but provide limited insight into the biological condition of activated sludge, the core component of secondary treatment. This article reviews contemporary biological methods for evaluating activated sludge properties, focusing on the diagnostic value of bioindication. It first outlines the activated sludge process, floc structure, key operational parameters (sludge age, organic loading, Sludge Volume Index, pH, temperature), and the ecological roles of indicator organisms such as ciliates, rotifers, nematodes, and filamentous bacteria. It then examines quantitative tools for assessing biocenosis structure, including classical biodiversity and dominance indices (Shannon, Simpson, Margalef, Pielou, McIntosh, Berger–Parker) and activated-sludge-specific indices, notably Madoni's Sludge Biotic Index, alongside methods for assessing filamentous bacteria abundance. These tools support prediction of disturbances such as bulking and foaming and complement physicochemical monitoring. The growing role of digital image analysis and machine learning in automating microscopic assessment is highlighted as an emerging direction. The review concludes that biological indicators offer a cost-effective, sustainable complement to chemical methods, and identifies the need for standardized interpretation of indices and further automation of microscopic diagnostics.