Welcome to Just Answer. Other things that can look like FIP are neoplasia, systemic mycoses (fungal infections), inflammatory bowel disease, hepatic disease, renal disease, retroviral diseases, and toxoplasmosis. A lot of times the wet form (where fluid in the abdomen is found) is diagnosed by the appearance of the fluid and the age of the pet.
The FIP 7b Elisa is a test that still is up there as far as how accurate it is.
Here is a very good link on FIP that I want you to have.
Unfortunatley if your little one has FIP, it is a very grave situation as there is really no cure for it.