When using a durometer make sure that the durometer is used on tires of the same temperature to insure relative measurements. Hot tires are going to be softer than cooler tires. You will notice that tires sitting in the hot sun are softer than those that are shaded. You can use a pyrometer to improve the consistency from durometer readings. You can also track the tire temps after a run.
New tires make more heat than used tires.
Worn out tires will not produce as much friction to produce tire heat. By tracking the temperature of the tires after a run, you will learn to anticipate when the temps are falling off too much. If you are diligent in your monitoring you will know when to hit the tire truck for that new set. You can also use a tread depth gauge to check the amount of tire wear. On many tires, the grip falls off dramatically at a given wear depth.