You should periodically check the bottom of your running shoes for wear...once the tread in any area looks worn away, it's probably time to start looking for new ones...and you should keep a tally somewhere of how many miles you have on your running shoes. I have a little dry erase board on my fridge where I keep a running total of the mileage on my shoes. I've read many different mileages that are considered the most you should have on a pair of shoes...anywhere from 300-500...I assume that number will be different for each runner depending on the types of surfaces they run on, their weight maybe, brand of shoe, etc.
And, while I'm on the subject...it's always been suggested to me that it's ideal to have more than one pair of running shoes at a time. One pair to use for shorter runs and another for long runs only. Once my short run shoes reach their max mileage (which is normally determined by hip pain for me

), I get a new pair to use for long runs, and what I was using for long runs I start using for my short runs. This way, the long run shoes are always the ones with the least amount of mileage and the most amount of tread and cushion left in them...which I certainly appreciate on a longer run.
If you aren't sure if the shoes you have now are "worn out" just take them in to a local running store and someone there should be able to check the wear on the bottom and suggest an approximate amount of miles they may have left on them for you (and they can normally tell you if you have good running form too just from looking at the wear pattern on the sole)...and from there you can get in the habbit of keeping track of the mileage on your shoes.