A reaction is redox only when at least one element changes oxidation state (electrons actually move); whole families of reaction never do, and naming the family answers the question faster than assigning every number.
Trigger: 'which of these equations represent(s) a redox reaction' or 'which involve(s) oxidation'.
Instances:
(i) Never redox: precipitation (ions pair up into a solid), neutralisation (acid with a base, alkali or carbonate), and combining a basic oxide with an acidic oxide into a salt; every element keeps its state.
(ii) Always redox: displacement (a more reactive element pushes out a less reactive one), disproportionation (one element goes both up and down at once), and decomposition of a compound into its free elements (state 0 on the right).
(iii) Gain of oxygen is oxidation and loss of oxygen is reduction, so a species that visibly picks up or sheds oxygen (CO to CO2, CO2 to CO) flags a redox change without any number crunching.
