A 28-year-old woman is brought to the trauma center after a stab wound to the left lower chest (5th intercostal space, midaxillary line). She is hemodynamically stable (BP 118/76, HR 92/min), has normal breath sounds bilaterally, and no respiratory distress. Chest X-ray shows a small left-sided pleural effusion but no pneumothorax. Which investigation is most appropriate to exclude hemothorax and guide the decision between observation and tube thoracostomy?