How can you tell whether a possessive pronoun is being used as an adjective?

A possessive pronoun functions as an adjective when it modifies a noun, indicating ownership or relationship. For example, in the phrase "her book," "her" is a possessive pronoun acting as an adjective because it describes the noun "book." If the pronoun stands alone without a noun (e.g., "That book is hers"), it is functioning as a possessive pronoun, not as an adjective.