Locating the Source of Your Leaking Roof:
Inspect for cracks, splits, tears, or any visible problems. If you find any, they most likely will represent a leak.
Inspect all roof penetrations (skylights, roof vents, etc.), to see if cement still adheres. Run a key down a vertical surface. See if cement comes away from the surface. Any separation indicates a roof leak. Mostly roof leaks occur at a vertical penetration of the roof.
Check all places where different materials come together - where metal meets brick, shingles meet metal, etc. These surfaces expand and contract at different rates, and therefore tend to pull away from each other. Check for tight seals. A loose seal might indicate the source of your leak.
If all else fails, try locating the leak from the inside. Take a flashlight into the attic, and try following the leak uphill. It will originate at some roof penetration. This will indicate your leak area.