My heart goes out to you, seeing your cat ill is difficult to take. We cried for days when our Moses became ill, then died.
The biggest signal that your cat has FIP is a bloated stomach or chest area. And only your vet can tell you for sure that is indeed FIP or something else. Basically, they puncture the skin and take a sample of the fluids. If it is stringy and yellow, it's a 95% chance he has FIP. The other possibility is liver disease. The liver can be repaired, though it is costly.
The symptoms our little moses had were:
-Went from being very active to inactive within the course of a week or so.
-He lost a ton of weight, yet his belly seemed big. Feel around your kitty on his spine and ribs. If those jut out at you, that's a warning sign.
-His belly suddenly got huge. I'd say it was over the course of two days his belly went from relatively normal looking to big. His belly swayed quite a bit when he walked.
-He started urinating uncontrollably and out of the box.
Other symptoms that I've heard are:
-Lack of appetite.
-Strange behavior, acting weird.
There are two forms of the FIP, wet and dry. Wet is most common and is mostly spread from an infected mother. Dry is much less common, and is a silent killer because often there are no visible symptoms. Obviously Moses had wet.
We have a few regrets about Moses. First off, our Vet had no homeopathic background at all. Had she, we might have been offered some more options to find a treatment for him. By the time we looked at that possibility, it was too late. We had Moses on antibiotics before he was diagnosed with FIP. This is another regret. The antibiotics probably weakened his immune system to the point where the virus could totally control him. Our biggest regret--getting angry at him for the urination around the house--this was before we knew what was wrong with him though. He was diagnosed on a Friday and died that Sunday night. We barely had any time to say good bye.
I noticed I forgot to give you that URL. Here it is. The best defense against these strange kitty diseases are education and preventative measures:http://meow.best.vwh.net/fip/
hope this helps!