That is a very generic question... If you're asking about the diagnosis & management or detection of pleural effusions, here's a good paragraph from Asbestos & Mesothelioma website

"The first step to diagnose pleural effusions is a chest x-ray that shows fluids building up around the pleural spaces. A physical examination of the patient is conducted on the basis of medical history. In order to diagnose the patient with pleural effusions, the medical doctor must be able to detect atleast 300mL of fluids using upright chest films. If the level of fluids exceeds 500mL, this indicates clinical signs in the patient such as diminished breath sounds, decreased vocal reflection, etc. Once a pleural effusion is diagnosed, the cause of it must be concluded and this is done via a thoracentesis. A needle is inserted through the chest wall into the pleural space to extract a sample fluid, which is then tested for the following properties:

i) Chemical compositions such as proteins, albumin, amylase, glucose and pH.
ii) Bacterial cells to detect any bacterial infections
iii) Count # of cells
iv) Cytology or the study of cells to identify any malignant cells
v) Other required tests to make a conclusive diagnosis"