PESTEL is probably needed if you are required to analyse the environment or a company’s strategic position. You can only use it if the question provides data for some of the headings.
5 Forces looks at competitive environment and can only be used if you are given information about at least some of the 5 forces.
Often both models can be used simultaneously in the same question – they are not mutually exclusive. You have to look at the information given in the question to see which model(s) it might partially fit.