Primary pericardial mesothelioma is often diagnosed after it has metastasized to lymph nodes or the lungs. Imaging
Diagnosing mesothelioma is often difficult because the symptoms are similar to those of a number of other conditions. Diagnosis begins with a review of the patients medical history. A history of exposure to asbestos may increase clinical suspicion for mesothelioma. A physical examination is performed, followed by chest X-ray and often lung function tests. The X-ray may reveal pleural thickening commonly seen after asbestos exposure and increases suspicion of mesothelioma. A CT (or CAT) scan or an MRI is usually performed. If a large amount of fluid is present, abnormal cells may be detected by cytopathology if this fluid is aspirated with a syringe. For pleural fluid, this is done by thoracentesis or tube thoracostomy (chest tube); for ascites, with paracentesis or ascitic drain; and for pericardial effusion with pericardiocentesis. While absence of malignant cells on cytology does not completely exclude mesothelioma, it makes it much more unlikely, especially if an alternative diagnosis can be made (e.g., tuberculosis, heart failure). However, with primary pericardial mesothelioma, pericardial fluid may not contain malignant cells and a tissue biopsy is more useful in diagnosis. Using conventional cytology  diagnosis of malignant mesothelioma is difficult, but immunohistochemistry has greatly enhanced the accuracy of cytology. Biopsy
Generally, a biopsy is needed to confirm a diagnosis of malignant mesothelioma. A doctor removes a sample of tissue for examination under a microscope by a pathologist.
Research
The WT-1 protein is overexpressed in mesothelioma and is being researched as a potential target for drugs.There are two high-confidence miRNAs that can potentially serve as biomarkers of asbestos exposure and malignant mesothelioma. Validation studies are needed to assess their relevance. References
This article includes information from a public domain U.S. National Cancer Institute fact sheet. External links

Mesothelioma at Curlie