Silicone does not stay as it was supplied. Heat, oil mist and time change its chemistry at the surface, and the first sign is that it stops feeling supple. That is a material change, and it happens well before anything leaks.
The question is whether the change has reached the parts of the hose that do the work. A hose can be stiff on the outside and still sound at the liner and the reinforcement, and that is the distinction worth investigating.
Squeeze the hose along its length rather than at one point. A healthy hose gives evenly and returns. A hardened one feels firm, and the change is often worse near the hot end, which tells you the exposure was one-sided.
A hose that is hard but uniform has aged. A hose that is hard in one place and soft in another has a local problem, usually heat or oil, and that is the one to worry about.
Fine surface cracks are the visible stage of the same process. Cracks that stay in the cover are cosmetic. Cracks that reach the reinforcement admit the medium to the fibres, and the reinforcement then fails by a different mechanism and fails quickly.
Bend the hose and look at the outside of the bend under a light. Cracks that open as the hose bends are deeper than they look when it is straight, and that is the test worth trusting over a glance with the hose at rest.
A hardened hose does not recover from being clamped. The wall takes a permanent set at the clamp, and the next clamp has to seal against a flat rather than a round. That is why a hardened hose often leaks at the clamp before it fails anywhere else.
Look for a groove under the clamp that stays when the clamp is removed. A performance silicone hose with a deep set at the bead has already lost the elasticity that made the joint work.
A hardened hose seals because the clamp is still providing the load that the material no longer provides itself. Move the clamp, reuse a different position, or lose a little tension and the joint starts to weep. The margin has been spent.
Where the medium is fuel vapour or oil mist, the liner matters as much as the wall. A fluorocarbon lined silicone hose resists that attack at the liner, which is why the lining choice affects how long a hose stays flexible in service.
Keep a hardened hose if the liner is intact, the reinforcement is sound, there is no deep clamp set and the joint it serves is a low-demand one. Replace it if any of those is doubtful, or if the hose has to come off anyway for other work.
The rule is about margin rather than appearance. A hose that is still working has margin left; one that only works because nothing has been disturbed has none, and that is the one to change before it chooses its own moment.
