My vets have also always said the only way to definitively diagnose IBD is via biopsy, but that was not an option for instance with Patrick given the age it came up and his overall health.
In his case, it was a presumptive diagnosis based on symptoms and the fact that on palpation, his intestines felt thickened and ropey...an ultra sound can also be used to check for any masses - which might mean something else is going on. I like U/S as they are non-invasive
In Patrick's case we treated as if versus (in his case) lymphoma, and based on his responding and how much longer he lived after this diagnosis, considered that it was indeed IBD.
Not sure this helps you very much, but this is some of what I've learned with my kitties (more than 1 has had this presumptive diagnosis in the past).