First look for where water sits
Ponding is useful information. It may show that the deck has sagged, the falls are poor or an outlet is partly blocked. Standing water is not automatically a leak, but it puts more pressure on laps and small defects.
Different coverings fail differently
Old mineral felt can crack and blister. EPDM is more likely to have trouble at seams, corners or poorly glued edges. GRP can crack at movement points or around trims. Calling every flat roof “felt” can lead to the wrong repair being discussed.
Edges and wall junctions
These are the first places I would inspect closely. A sound central area can still leak badly if the upstand against a wall is too low or the edge trim has opened.
Patch or renew?
A local patch makes sense where the deck and surrounding covering are sound. If the roof is brittle, heavily blistered or has several old patches, another small repair may buy very little time. The decision should be based on condition, not age alone.