A 58-year-old man undergoes elective total abdominal hysterectomy under general anesthesia. Postoperatively, he is enrolled in IV-PCA with morphine (1 mg bolus, 10-minute lockout) versus a colleague's patient on EPCA with morphine 0.1 mg/mL + bupivacaine 0.1% (2 mL bolus, 15-minute lockout). Which clinical finding would most reliably distinguish the two analgesic modalities in the first 24 hours?