Newer homes 30 years old or less usually don't require any repairs unless the roof has been leaking quite a while. Homes that are between 35 years old and up usually require at least some repairs. The older the building, the more repairs that are likely necessary.
Careful examination of the roof can reveal that at least some repair work is needed; however, It is difficult to exactly judge how much repairs are required until the roofing is removed (another reason to never put additional layers on a roof).
Surprises are possible. Roofing is not a perfect science. An experienced roofer and an inexperienced roofer may be looking at the same thing, but they do not (see) the same thing. Tearoff is always the best way to go.
It is cheaper in the long-run to do repairs during the re-roofing process than it is to return after-the-fact and dig into the structure.
