Automatic verification tools such as COVER have proven to be valuable aids in the validation process for knowledge-based systems (KBS). COVER checks KBS for logical anomalies. Background domain knowledge can allow COVER to detect errors in knowledge bases that would otherwise go undetected. Ontologies are a necessary component of knowledge sharing: two KBS cannot share knowledge unless they commit to a common ontology. Ontologies also provide a rich source of background domain knowledge for validation. This paper describes a tool, DISCOVER, which verifies KBS against ontologies. DISCOVER verifies heterogeneous sources of knowledge: KBS are represented in CRL (COVER rule language), and ontologies are represented in MOVES (Meta-ontology for the verification of expert systems). The paper describes MOVES and CRL, and discusses a number of anomalies arising between KBS and ontologies. It is shown that DISCOVER can be used to verify that a KBS commits to a given ontology, which is a prerequisite for sharing its knowledge.