There are many things that can mimic UC's pattern of inflammation, ulcerations and symptoms.
Ulcers aren't always present in every UC patient because there are varying degrees of inflammation and symptoms, but for some, ulcerations are in other bowel afflictions as well. So, it's not always the only determining factor.
It's definitely a process of elimination...which my GI did during diagnosis. UC was the first assumption and the last on the cross-off list.
I found this site...might be helpful.