Devices and methods for treating obstructive sleep apnea by first performing an assessment of the patient that involves observing the patient's upper airway during a tongue protrusion maneuver. The assessment may, for example, be done using endoscopy to observe the upper airway while the patient is awake in the supine position. An adequate response of the upper airway during the tongue protrusion maneuver is indicative of likely therapeutic success with hypoglossal nerve stimulation, and may be used for making clinical decisions. The principles of the present invention may be applied to other therapeutic interventions for OSA involving the upper airway.