There is a very simple way each person can measure their conditioning level (cardio/pulmonary fitness).
If one is a runner for example, they can do something like the following; warm up by running a mile and then run a 1/4 mile as hard as you can. Immediately upon finishing, take your heart rate and note it. At 30 seconds post exercise, take it again and do the same at 60 seconds post exercise and not the changes.
As one continues to train over the weeks, they can do this test once or twice a week and not the results.
What are you looking for? The better condition you are in, will cause the post exercise heart rate to drop more quickly, if one is not in good condition, the heart rate will drip much more slowly. When the heart rate recovers more quickly, it simply means the cardio pulmonary system, is able to better deliver the oxygen to the muscles and rid the body of Co2 which builds up during exercise. The quicker your heart rate recovers also means, you will be able to run faster and farther, without undue fatigue.