How do I know if my gutters need replacing or just repairing?

Start by separating three jobs people lump together: cleaning, repair and full replacement. A gutter that overflows in heavy rain has often just filled with leaves and moss, so clear it and check the fall first — a blockage or a dropped bracket will mimic a failing system and cost you nothing to put right.
Repair is the right call when the fault is local: one leaking joint, a cracked length, a slipped bracket or a downpipe that has come apart. On a modern uPVC system these clip-together parts are cheap and quick to swap, so replacing a single section is the sensible fix.
Lean towards full replacement when the faults are spread out or the material is simply old. uPVC guttering typically lasts 15 to 25 years before it goes brittle and the joints stop sealing; aluminium and steel around 20 to 25; cast iron can last far longer but is due for replacement once rust is widespread. The tell-tale signs that a whole run is finished: several joints weeping at once, sagging that returns after re-fixing the brackets, water staining or damp on the fascia and wall behind, and plastic that cracks when you flex it.
Check one thing at the same time. If the fascia or soffit boards behind the gutter are soft or rotten, new guttering bolted to failing timber will only sag again, so budget to sort both together.
If it is more than a quick clean or a single joint, get a roofer or guttering specialist to take a proper look before deciding — a diagnosis is usually free as part of a quote. For typical prices see our guide to gutter installation costs. When you are ready, BestBuilders can match you with vetted local gutter installers for free, no-obligation quotes.