Hot water cylinders often give warning signs before they fail completely. Catching those signs early gives you more choice and less mess.
Look for leaks and rust staining
Damp patches, rust marks and water where it should not be are the usual early signs.
Notice when hot water is running out sooner
If you are getting less hot water than you used to, the cylinder or immersion setup may already be struggling.
Decide whether repair or replacement makes more sense
Sometimes the right answer is a simple repair. Sometimes the cylinder is telling you clearly that it is time to replace it. The key is getting an honest assessment before it fails on its own terms.