How to diagnose IBD is a long story! We did it by symptoms and an ultrasound that showed structural changes in the intestine that to experienced veterinary eyes were consistent with moderate to severe IBD. In his case it was a no-brainer, given everything we saw. However, orthodox veterinary thinking is that IBD can only be definitely diagnosed by either an endoscopic or surgical biopsy, which will reveal what type of inflammatory cell(s) is(are) involved. Our dog was already so sick and had lost so much weight, and was an old 8 years old, that the vet and I agreed that putting him through that procedure wasn't worth it, especially because the knowledge gained wouldn't change the treatment much if at all. It's treated mainly with metronidazole, tylosin, feeding a novel protein exclusively, and an immunosuppressant (prednisone or budesonide).
Hopefully, you won't need to go there!
ETA: Eating poop, cat or otherwise, is likely to indicate nutritional inadequacy, perhaps because of something like Intestinal Malabsorption. With IM comes a loss of ability process B-vitamins in the gut and the need to give B-12 shots monthly for life. There is a simple blood test test for it. If it'spositive, you then have to ask what caused the IM, and then you're looking at possible intestinal infection or IBD as likely culprits. And all of this seems to develop more often in dogs with a history of hookworm, as near as I can tell, especially if the hookworm wasn't diagnosed promptly or treated aggressively.