Age comes first. Many asphalt shingle roofs in Central Florida start to struggle somewhere between fifteen and twenty years, earlier if the attic runs hot. If your roof was here when you bought the house and you don't know its age, that's reason enough for an inspection.
Second and third: granules in the gutters and shingles that curl, cup, or crack. The granules protect shingles from the sun, and once they wash away, the shingles break down quickly. Fourth: moss, algae, or dark streaks that won't go away, which hold moisture against the roof.
Fifth: a roof that sags or feels soft underfoot, which usually means the decking underneath is wet or rotting. Sixth: water stains on ceilings or in the attic, even small ones. Seventh: rising cooling bills, which can point to poor attic ventilation cooking the roof from below.
One sign on its own doesn't always mean replacement. A free attic and roof inspection tells you which problems are cosmetic and which aren't, and if your home qualifies for a new roof, you may get up to $10,000 off your first project.