Non-invasive ventilation (NIV) has become a key treatment for the management of chronic hypercapnic respiratory failure, particularly in patients with chronic obstructive pulmonary disease, obesity hypoventilation syndrome, and neuromuscular disorders. Over the past decades, its use has expanded well beyond its initial scope, and the assessment of patients on NIV now extends beyond technical settings or blood gas measurements alone. This article reviews the main tools available for evaluating the effectiveness and tolerability of NIV and presents a structured, step-by-step clinical framework - summarized by the acronym FOLLOW- to guide this evaluation.