Chronic heart failure is a global health issue and a leading cause of morbidity and mortality. As its prevalence increases, it imposes a growing burden on both individuals and healthcare systems. Patients’ adherence to optimal drug treatment is crucial for achieving the best possible outcomes and effectiveness. However, medication adherence rates vary widely among patients with heart failure, and thus it is essential to accurately assess patients’ adherence. In this comprehensive review, we aim to provide insights into the practical aspects of various methods of adherence assessment. Direct methods include directly observed medication intake, serum drug level measurement, and biological marker testing. Indirect methods can be divided into subjective methods (patient–physician dialogue, patient questionnaires, and patient diaries), and objective methods (pill counts, pharmacy refill records, and electronic monitoring devices). Healthcare professionals must recognize and understand these different methods to develop strategies that optimize patient outcomes, and mitigate the economic implications associated with heart failure.