Welcome to the gastroenterology community! I don't believe a CT scan can show an ulcer. If you are worried about an ulcer you should talk to a GI doctor about getting an endoscopy. What symptoms are you having?
or a colonoscopy (lower endoscopy), then it will not show a ulcer. If by upper GI, you mean an upper GI series, I believe it will not always show an ulcer. If by upper GI, you mean an upper endoscopy, then yes it will show an ulcer.