There are several indicators:
1. Press the center of the sole down with your thumb. If it's soft or spongy, it's time.
2. When you see circular patterns of wear in the center of the sole.
3. Check the inside and outside borders of the sole laterally across the area where the ball of your foot would be. If the sole is thin and close to the welt, it's time.
4. If moisture begins to seep in through the bottom of the sole on damp days.