The soles of your shoes can tell you a lot about how you walk.
Take a well-worn shoe and examine the wear pattern on the sole. This will quickly indicate how your feet strike the ground and how your feet move as you complete each step. Shoes with some tread (hiking boots, running shoes) are better as the wear pattern is usually more apparent.
As you walk, your foot pronates—the weight rolls from the outside of your foot toward the inside of your foot. How much pronation occurs is readily revealed by the wear pattern on the sole.
Over-pronation occurs when most of the weight is transferred to the inside of the foot, particularly the forefoot. Under-pronation, or supination, occurs when most of the weight remains on the outside of the foot. Over-pronation is more common than supination, especially among individuals with low arches (a.k.a. flat feet).
There is a spectrum between the two. A neutral gait falls roughly in the middle, without excessive weight transfer from the outside to inside.
The wear pattern on your shoe sole provides a clear indication of where you fit in this spectrum. Lots of wear along the inner edge your sole, especially in the forefoot area? Over-pronation. Along the outer edge? Supination.
Why is this information useful? The vast majority of people walk normally, with little to no risk, because they fall somewhere near the middle of the supination-pronation spectrum.
There are some risks, however, at the far ends of the spectrum. Over-pronation and over-supination can increase the risk of ankle sprains, and have potential longer-term implications for the knees and hips.
Even if you’re not at the far end of the spectrum, however, keep in mind that as your shoes wear, they will reinforce and exacerbate your natural walking tendencies. For example, as the back corner of your heel wears down, it changes the angle at which your foot strikes the ground and can increase the risk of twisting an ankle.
Indeed, wear on the corner of my heels is often my primary motivation to retire the shoes. (If you have hiking boots you love, consider getting them resoled, which costs less than a new pair.)