How do you know if the timing belt is going out?

What I was told is that you look at the belt; if you see more than 7 "cracks" per inch, it is time to replace the timing belt.

A timing belt can look perfectly fine and be ready to break. This is why they are replaced at a scheduled mileage or every 10 years no matter what they look like. So the bottom line is you cannot know when it needs replacing by just looking at it. Your owners manual will list when it needs replacing.