Short answer
Measure resting voltage after several hours off charge, then measure again while cranking. A healthy 12 V battery rests near 12.6 V and should not collapse below roughly 9.6 V under cranking. A large drop with good resting voltage points to a failing battery; no drop with no crank points to connections or the starter circuit.
You will need: Multimeter, spanner, wire brush, safety glasses
Most "dead battery" calls turn out to be connections. The test that separates the two takes ten minutes and a multimeter.
The two measurements
Resting voltage, measured at the battery posts after several hours off charge and with no load. This tells you state of charge, not health.
Cranking voltage, measured at the posts while someone cranks the engine. This tells you whether the battery can deliver current. A battery that rests well but collapses under cranking is failing internally.
Figures vary by chemistry — lithium and AGM behave differently from flooded lead-acid — so compare against your battery's own specification rather than a general rule.
Separating the battery from the circuit
If the battery tests well but the engine will not crank, the fault is in the path. Measure voltage drop across each connection while cranking: battery post to cable, cable to isolator, isolator to starter solenoid, and the earth return. A significant drop across any one joint is your fault. See how to find a voltage drop and will not crank.
Safety
Batteries produce explosive gas and can deliver enormous current into a dropped spanner. Remove rings and watches, work with insulated tools, wear eye protection, and never break a high-current connection under load. Anything involving 230 V AC charging equipment is licensed work.