While there is not a single test that shows whether or not a cat has FIP, you can have bloodwork done to check titers..or levels of antibodies. Now, this doesn't without a doubt rule OUT FIP if levels are low..nor does it without a doubt prove FIP with HIGH levels..but your vet can talk to you about how very high levels can show that there seems to be FIP present. A cat does not have to be very ill for levels to show. Some cats are carriers and have hardly any symptoms but can show elevated levels. Some are carriers and have low levels..some are mildly sick and have levels off the charts. Our vet sends blood to a state lab, the best in the state, to check these titers. While there is no treatment for FIP, there is supportive care. Some cats don't make it as long..some can go on for awhile and not be as affected by the disease for some time. You mentioned your cat having IBS.. vomitting is a sign of FIP if your vet hasn't mentioned that already. Sadly, while many view FIP as rare...it's not so much anymore. It's just that more vets, for whatever reason, don't talk about it as much and many don't think of it as much of a threat, say compared to FIV or FelV.. but in reality, while not COMMON in most places, it's definitely not uncommon either.
Anyway..kudos to you for giving her a good home she's gorgeous..and best wishes with her continued health.