How to tell when a new roof is needed?

[Ross Bernard]

How do you determine when a new roof is needed on one of your properties? The roof on our two story home is 15 years old, 3 tab asphalt shingle. I have noticed several neighbors have replaced their roof this year. The roof has a fairly steep pitch and is on the second floor, so it is not easy to access safely. If I have a roofing contractor inspect the roof, isn't he likely to tell me it needs to be replaced regardless? My trust of contractors is not very high.