1. The key sticks or catches

If you have to jiggle the key every time, the pin stack or cam is wearing. Left alone it will eventually snap in the barrel.

2. The handle feels loose or droops

Play in the handle usually means the spindle or spring cassette inside the lock body is worn.

3. Visible rust or corrosion

Surface rust on the faceplate is cosmetic; internal corrosion is not and will seize the mechanism.

4. It no longer meets the current standard

Older cylinders may not carry the anti-snap markings insurers now expect.

5. It has been on the door for 10+ years

Mechanisms wear. A decade is a sensible replacement window for a front-door lock.