Jan is correct, they can do a million tests and have them all negative for Crohn's and you could still be found to have Crohn's after J Pouch surgery. No matter how certain they can try and be, there is no pure or absolute certainty.
Keep in mind I have been negative on a CT Enterography and on a Prometheus Blood Test for both Crohn's and UC. Yet I have chronic inflammation in my pouch and in my ileum above the pouch since 2008.
Basically the doctors do their best to try to confirm a UC diagnosis and then you roll the dice and hope they are correct. But if the entire colon is inflamed and has to come out, the diagnosis is irrelevant in terms of the decision to have a colectomy as you are just waiting for a cancer diagnosis or a perforation. I had dysplasia in my colon and it was dissolving in my surgeon's hands when he removed it so the possibility of me having been a Crohn's candidate at that time, and using that to deny the surgery, likely would have caused my death some time in the 1990s. However both my GI and my surgeon were quite certain of a UC diagnosis and there was no evidence at that time of any inflammation in my ileum or other sign of Crohn's Disease.