A musty smell that shows up on the top floor before you can see anything is the most common first clue. Dark staining around roof nails, discoloration on the underside of the decking, or insulation that looks matted and gray instead of fluffy are the next signs, usually found during a home inspection, an insurance claim, or a trip up to fix a wire.
Coastal humidity is the usual driver. Soffit vents pull in warm, wet salt air all summer, and if the ridge vent is undersized or blocked, that air sits against the roof deck instead of moving through. A roof leak after a tropical storm speeds the whole process up, and ductwork running through the attic sweats in the heat just like a cold glass on a porch table. Waiting rarely helps: insulation holds moisture longer than open wood, and once the growth reaches the trusses or the decking, treating it gets more involved.
If your home just went through a re-roofing job or a named storm blew through the Grand Strand, it's worth a look up top even before any smell shows up. New roofing work sometimes disturbs old flashing around a chimney or vent pipe, and wind-driven rain can push water past shingles that looked fine the week before.