One of the ways doctors diagnose peritoneal mesothelioma is by looking inside the abdominal cavity with an instrument called peritoneoscope. In this procedure, a cut is made through the abdominal wall and the device is inserted into the abdomen. This test, called peritoneoscopy, takes place usually in the hospital.

Some patients develop excessive fluid in the abdomen. This is called an effusion. The doctor could take a sample of fluid to diagnose if there is peritoneal mesothelioma. The fluid in the abdomen also may be drained to relieve symptoms of mesothelioma of the peritoneum. This fluid drainage procedure known as paracentesis.