Because there are so many different scenarios for electrons transfer, we have so far focused on only a few:

metal displacement
hydrogen displacement
halogen displacement
combustion
non-trivial redox

Generally speaking, if a reaction does not fit the models we have established for precipitation and acid reactions, it is likely to be redox. But the only sure way to determine whether a reaction is redox or not is to check for a transfer of electrons.

Oxidation numbers are useful for exactly this kind of task.