Can one tell the difference between diverticulitis and ischemic bowel on CT?

Sometimes. Colitis of splenic flexure (watershed area) suggests ischemia. Colitis of of rectosigmoid region suggests diverticulitis. However, these are general guidelines and do not always hold true. Consider clinical scenerio and other studies to definitively distinguish the two entities.

Source

Corman, Marvin, MD. Colon and Rectal Surgery. Philidelphia: Lippincott Williams and Wilkins, 2005.