First of all, check the roof surfaces and inspect for evidence of excessive shingle wear in one or more areas. Common problems of an aging roof are blisters (raised “bubbles” in the shingles), loss of granules, or curling.
Next, check for “soft spots” on the roof where water damage may have caused the roof deck to rot. This can be found in valleys (where two roof surfaces meet and form a “V”), areas near flashing or penetrations, or where there is a nail hole. Nail holes can be from scaffolding used by siding installers (if they don’t caulk them when they are finished), or nails that have popped loose from the roof installation. These are slow leaks that require only a minor repair if they are caught early enough.
Finally, check all roof penetrations and flashings to be sure they are secure and watertight. Look for evidence of damage to the shingles or surrounding roof area, and along walls and chimneys to be sure flashing is secure.