Are you improving? Check the numbers.

This requires a heart rate monitor and a treadmill or other easily replicated output.

Create a series of intervals which you are capable of replicating. Such as:

Treadmill
3 minutes 4.5 mph
3 minutes 5mph
3 minutes 5.5 mph
3 minutes 6 mph
1 minute Stopped or active recovery.

Don't stop until test is over.
At the end of each stage. note your HR at that moment, or if your watch does laps, you might be able to get averages for each stage. Add all of these together and you have a total score which you compare to as you go along. It's suggested that you try to lead into this test similarly each time (after a rest day for example). Performed at the same of day... for the sake of "control".

Obviously, your score should decrease as your aerobic fitness improves.