Cytotoxic anticancer treatment may induce amenorrhea or menopause to a variable extent. These side effects may not only impair or impede fertility but also cause sexual dysfunction, bone loss, and menopausal symptoms, with a strikingly negative effect on quality of life in many women. Aromatase inhibitors (AIs) are a recommended adjuvant endocrine treatment option in postmenopausal patients affected by early breast cancer (EBC) but are contraindicated in premenopausal women and in those with residual ovarian function. Women over 40 years of age with chemotherapy-induced amenorrhea (CIA) and routine hormonal levels consistent with menopause may receive an AI as adjuvant endocrine treatment. For these women, the tools available to identify menopause do not appear to be completely reliable. This review focused on the pathophysiology of ovarian toxicity induced by cytotoxic agents and on potentially useful methods to diagnose chemotherapy-induced menopause in patients treated with adjuvant chemotherapy for endocrine-responsive EBC. Moreover, practical approaches are proposed to distinguish true menopausal women, who would benefit from AIs, from those with transient or persistent CIA.