It can be difficult to recognize the signs of wear simply by looking at the shoe. Outsoles are so durable these days that the shoe's cushioning may be long gone before the tread is significantly worn down. The best test is just to pay attention to how you feel. It's a good idea to have more than one pair of shoes. Buy your second pair about midway through the life of your first, and alternate pairs.
Shoes wear down very gradually, and it can be difficult to recognize it without a reference point.
With a second, newer pair of shoes, you will feel the difference in cushioning when one pair wears out. This should make it easier to tell when it's time to bid the old shoes farewell and buy another pair. Do not, however, alternate between a brand new pair and a worn out pair. You'll only mix up your feet and cause injury. And you might actually squeeze a few extra miles out of them, too.