When I look at the two sentences below how can I tell straight away whether they are a reflexive action or a reciprocal action as they both have 'se' in the sentence:
Elles se maquillent
Elles se parlent
You need to look at the meanings of the verbs.
Elles se maquillent : reflexive because we assume each girl is making herself up.
Elles se parlent : reciprocal because we assume there are two or more of them and they're talking to each other/one another.
Obviously context is important, but to change from reciprocal to reflexive (or vice versa) we'd usually have something to give that context.
Elles se maquillent l'une l'autre. -> reciprocal (could be complicated simultaneously but we can imagine they do it in turns)
Elles se parlent toujours toutes seules ! « Toutes seules » makes the difference : each one is talking to herself -> reflexive.