Has left-sided uc and minor hemorrhoid. Is there a way to know if the bleeding is caused by inflamed colon or hemorrhoid?

Colonoscopy. The best way is to see where the bleeding is originating. You can have bleeding from hemorrhoids and from other problems including colitis, polyps, diverticulitis, and cancer.
Source of Bleeding. It could be one source or another and it could even be both sources. Very hard to distinguish one source from another. Ulcerative colitis oftentimes has inflamed tissue that may leak a little bit of blood and you may have internal hemorrhoids. Seek care if you have pain and cramping or fevers.