With a Shingle Roof, the shingles tend to wear out in a discreet way. Granules start coming off leaving black patches showing. Shingles will also start to curl at the ends.
In General, some people think if a roof leaks they need to replace it. Unless it’s an older roof, leaks can usually be repaired. With leaking in older roofs and/or if you roof has leaking in more than a few areas, this can mean trouble.