One way doctors diagnose peritoneal mesothelioma is by looking inside the abdominal cavity with an instrument called a peritoneoscope. In this procedure, a cut is made through the abdomen wall and the peritoneoscope is placed into the abdomen. This test, called a peritoneoscopy, is usually performed in the hospital. Some patients develop excessive fluid in the abdomen. This is called an effusion or ascites. A doctor may take a sample of such fluid to diagnose peritoneal mesothelioma.