A big part of the Maffetone method of endurance training is the maximum aerobic function test, or MAF test. Its stated purpose is to measure progress and to monitor for potential problems.
You should probably just read the link from above, but in short, it's a periodic test where one measures performance at the target heart rate provided by the 180 formula. A runner, for example, might run three miles, recording lap times of each mile. This is repeated monthly. Each lap within one test should get progressively slower, but from test to test, there should be improvement.
By comparing MAF test results over time, you can see progress. If progress halts or even reverses, that indicates some sort of problem, like under- or over-training, poor diet, or stress.
I don't do MAF tests. I have never done one. Here's the reason: the way I'm doing it, every day is a MAF test. I start at a pace that I can't maintain for the entire run at my target heart rate. My heart-rate-controlled treadmill makes sure I keep my heart rate steady, and adjusts my pace accordingly. When I've improved so I can maintain my starting pace at my target heart rate, I increase my starting pace. If that pace turns out to be a little too fast, my treadmill program takes care of it.
I chart everything over time, so I can see improvement. I watch my pace in particular. Earlier this year I was still improving as I drifted away from the regimen. I was pretty disciplined for around four months, and I saw my training pace improve by about 74 seconds per km, or two minutes per mile, while my heart rate actually slowed by about 5 bpm. It's hard to argue with results like that.
Summer rolled around, I abandoned the treadmill for the road, and I stopped training by heart rate. I mostly didn't even measure heart rate.
I started this in February; this time around I've started in December. I'm curious to see what improvements I'll get during treadmill season. I also hope to extend it to the outdoors. I have the necessary equipment; now I just need to do it.