Catching small problems early is the difference between a coating and a full replacement.
Ponding water that sits for more than a day or two after rain is the clearest warning sign on a flat roof.
Look for chalky, powdery surfaces, cracked or lifting seams, rust streaks around fasteners, and stains on interior ceilings.
If you see any of these, a quick inspection now usually keeps you in coating territory instead of replacement territory.