There are two main ways to check an oxygen level. If you are talking about an oxygen saturation, this only tells you the percentage of hemoglobin bound up by an
oxygen molecule in your blood. The higher the number, the more oxygen you are carrying in your blood. This gives a picture of how much oxygen you are breathing in, but it cannot really tell how severe your lung disease has become. For example, many people with emphysema will still feel short of breath even with a good oxygen saturation.
There is also another way to check your oxygen level in your blood. This is called an ABG, or arterial blood gas. This blood test is very different from most blood tests, as it is obtained using a blood draw from an artery, not a vein. The most common site for this test is the radial artery in the wrist. With this test, we can get the results for partial pressures of oxygen and carbon dioxide in the blood. The testing is used to determine gas exchange levels in the blood related to lung function when peolple are very ill, so it is not a test that is used to routinely check lung function.
The best way to determine your lung function would be to have a pulmonary function test. This is a multipart test that measures your breathing compared to what people should normally be able to do. This will tell you exactly how severe your disease is, rather than relying on oxygen levels.