Final answer:
A typical visual indicator that oxygen is flowing in an oxygen mask is the inflation of the reservoir bag. A pulse oximeter is a different device that can also measure oxygen levels by showing the oxygen saturation in the blood.
Step-by-step explanation:
The visual indicator on a mask that shows oxygen is flowing can vary depending on the type of mask or delivery system in use. However, generally, a common indicator is the presence of reservoir bag inflation. In oxygen masks equipped with a reservoir bag, when the oxygen is flowing properly, the bag will partially inflate with the oxygen. This visual cue assures health professionals and patients that oxygen is being supplied. Another related device, although not a mask, is the pulse oximeter, which measures oxygen saturation levels.