Diagnosis of FIP is based on history, clinical signs (see “Signs and Symptoms”), and laboratory tests. Blood tests only demonstrate if a cat has been exposed to FCoV and cannot distinguish between FCoV and FIP.  Effusive and Noneffusive FIP require slightly different diagnostic methods, but if the criteria for diagnosis are precisely met the probability of proper diagnosis is high.
Video of cat with ascites (source: http://www.lbah.com/feline/fip.html)
Effusive FIP
Diagnosis is based on:
-    Analysis of fluid exudates in the peritoneal space:
-    fluid is sterile, viscous or ropey, and yellow to tan; may contain fibrin strands
-    high specific gravity (1.017 – 1.047) and high protein content (5-12g/dL)
-    Wright’s stain = mixtures of neutrophils, lymphocytes, macrophages, and fewer mesothelial cells in a granular, eosinophilic, proteinaceous background.
-    exudates with total protein >3.5 g/dL (of which >50% is gamma globulin) and cytology consistent with FIP are very indicative of FIP.
-    albumin: globulin ratio <0.45 predicts effusive FIP
-    positive titre can support diagnosis of effusive FIP, but it is possible to carry FIP and be asymptomatic
- PCR:
-    identification of coronavirus RNA by reverse-transcriptase PCR, also supports diagnosis
Non Effusive FIP
Diagnosis is based on:
-    Clinical signs:
o    nonresponsive fever, weight loss, multisystemic signs (including ocular and CNS), and increased coronavirus titers in cats
-    Lymphopenia
-    Hyperglobulinemia (>5.1 g/dL)

If these three features (clinical signs, lymphopenia, and hyperglobinemia) are observed collectively there is a 90% probability of having FIP.  
-    Other laboratory tests that may be helpful:
o    clinical chemistry indicating liver, kidney or pancreas dysfunction.
o    CSF analysis, in neurologic cases, showing increased protein content and neutrophil numbers
o    laparotomy and organ punch biopsy of lesions = histopathologic changes