1) Hypoxemia—until proven otherwise.
2) Tongue, lip, toe web, vulva, prepuce, flank fold, and pinna.
3) Check the patient first. If in doubt, give a breath. Check the pulse and auscultate the lungs. Then check the pulse oximeter, the anesthesia circuit, including the placement and patency of the endotracheal tube, as well as the oxygen level in the tank. Test the probe on your pinky.