How do you know if it is reflexive or intensive pronouns?

A reflexive pronoun is used to 'reflect back' to its antecedent.

An intensive pronoun is used to emphasize its antecedent.

Examples:

Kim made herself a sandwich. (reflexive)

Kim answered the door herself. (intensive)

The children imagined themselves on a trip to the moon. (reflexive)

The children themselves cleaned the playground. (intensive)