Would an abdominal ultrasound show a tumor in the colon? does it even check the intestines?

No: Ultrasound does not even attempt to evaluate bowel. The gas inside the bowel does not allow the sound waves to penetrate, causing "shadowing" which makes it near impossible to even get a look at most of the bowel. Occasionally in very thin people or children, the appendix can be seen if it is abnormal, but otherwise ultrasound is useless for evaluating bowel.

Sometimes: Abdominal ultrasound typically examines the solid organs like, liver, spleen, ovaries, kidneys, however, if there is a solid mass in the intestine or dilated bowel, the ultrasound may identify these changes.