How does stress test check or measure oxygenation?

CPET. Cardiopulmonary exercise testing measures oxygen consumption at rest and during maximum exercise, usually on a stationary bicycyle or sometimes on a treadmill ...
Speciali equipment. An average stress test does not directly measure oxygen consumption. Specialized equipment used with a treadmill (GXT) test can measure oxygen consumption often reffered to as a "v dot o two" measure. A standard GXT measure workload achieved by how far you go and gives a "metabolic equivalent" or "met" or average expenditure to achive that workload. The higher the mets achieved, the better.