Just like your car when you take it for a service and MOT to make sure it is working properly, you should do the same with your running trainers. Did you know as a rule of thumb you should be changing your running trainers every 300-400 miles you run in them. This is because your running style is suited to your style of footwear. The more you exercise and run in the trainers it will result in wear and tear of the shoe.
If you think about it, everyone runs in a different way. The way the foot is planted to the way you push off from it. It could be on the side of the foot or on the inside. The more weight that is put on the trainer the quicker you will wear it out and the less comfort and support it will give you. Have a look at your trainer and see which side of it is worn, this will give a good indication on your running style. Be sure to keep your running shoes in check to prevent injuries :)