Final answer:
Investigators can reference FDA guidance documents and scientific literature to determine if an IND is required for a study of an approved drug.
Step-by-step explanation:
Investigators can reference various sources to determine if an IND (Investigational New Drug) is required for a study of an approved drug. One important resource is the FDA's guidance documents, such as the Guidance for Industry, CGMP for Phase I investigational drugs. These guidelines provide information on when an IND is required and the regulatory requirements for conducting clinical trials. Additionally, researchers can refer to scientific literature, such as peer-reviewed articles and publications, to gain insights from previous studies and regulatory experiences.