How do i tell if a substance has been Oxidised or Reduced?

We know from GCSE that Gaining of electrons is reduction; and Loss of electrons is oxidation. This can be remembered by the pneumonic: 'OIL RIG' :Oxidation Is Loss Reduction Is Gain.

At A-level it can be confusing which species has lost electrons and which has gained them so we calculate oxidation numbers on the left hand side of the reaction and on the right hand side. If a species is oxidised then the oxidation number will increase and it will decrease if a species is reduced.