Chalking
A powdery white bloom on the jacket is the earliest sign. It means the surface has degraded but says little about the insulation underneath, and many chalked reels are still placeable.
Cracking and crazing
Fine cracks that open when the cable is flexed are more serious. Once the jacket cracks through, moisture reaches the insulation and the reel moves toward recovery.
Heat deformation
Flattened or glossy patches where cable touched a hot steel flange indicate thermal softening. Deformed insulation cannot be trusted even if the jacket looks intact.
Photograph it in daylight
Sun damage is hard to see under floodlight. Photograph the outermost layer in shade, close enough to show texture, and note how many summers the reel has been outside.