A lesson from Yale's Addiction Treatment: Clinical Skills for Healthcare Providers course, part of a series taught by seven Yale instructors using a model case performed by actors. This installment covers how clinicians track whether addiction treatment is working, focusing on drug toxicology testing and patient engagement. It compares test matrices, urine, saliva, blood, hair, and sweat, weighing turnaround time, invasiveness, and cost against accuracy. The instructors flag the risk of false positives and stress reading toxicology results alongside a patient's own account rather than treating a screen as the final word. The central argument is about framing: drug testing works best not as a punitive checkpoint but as a tool for honest conversation between provider and patient about relapse and progress. Aimed at healthcare providers managing patients with substance use disorder, the lesson is short and practical, built around applying testing results to real treatment decisions rather than abstract theory.