Diagnosing peritoneal mesothelioma typically involves a combination of medical history review, physical examination, imaging tests (such as CT scans and MRIs), and biopsies. A biopsy, where a small sample of tissue is taken for examination under a microscope, is often necessary to confirm the presence of mesothelioma cells. Additionally, medical professionals may perform other tests to determine the extent of the cancer and its impact on surrounding tissues.