Often you cannot tell by looking at the shoes. The outsoles and uppers may appear just fine, but the midsole will lose much of its support and cushioning between about 300 and 500 miles (exact amount depending on the shoe, your running style, running conditions, etc.) This midsole damage is not always clearly visible, but you will likely notice the difference when running in new shoes vs. older ones of the same model. Keeping track of how many miles you have on the shoes is probably your most reliable option.

I have always stuck with the rule of 500 miles or 6 months, whichever comes first.

This is based upon my training plan of 20 miles a week

20 miles/week = 80 miles a month

480 miles in 6 months

I do have 2 pair of running shoes, 1 for training and 1 for races.  When my training pair wears out, I replace them with my race pair.

I then break in my new race pair for about a week doing my training runs.

This has worked for me, so far.  You have to figure out what works best for you and the type of shoe and running style you have. 

I hope this helps

I usually go more by time.  If I've worn the same pair of running shoes for 6 months, it's time for a new pair.  I also use knee soreness to gauge it.  If my knees are sore after two consecutive runs, it's time to get a new pair. 

You can also look at the soles to see how worn down they are but some shoes will wear faster than others so mileage might not be the best measure of a shoe's lifespan.