How can I tell if I have diverticulitis?

See a doctor NOW. Classic presentation is fever, abdominal pain (usually left sided), and elevated white blood cell count. Some people don't have the classic presentation. Other things, like appendicitis, pelvic inflammatory disease can present similarly . Need to see a doc, get examined, maybe get a ct scan. Mild cases- rx antibiotics as outpt;more severe -inpt rx, and/or drainage of pus, or surgery.
Left lower abd pain. If you suspect you have diverticulitis you should consult with your doctor as you will need evaluation and antibiotics. Generally symptoms consist of left lower abdominal pain. More severe cases may have fever, worsening pain, abdominal distention and vomiting.
Based on symptoms. The most common presenting symptoms of diverticulitis are abdominal pain, nausea and vomiting, constipation, diarrhea, flatulence, and bloating. The problem is that these symptoms can be seen with many gastrointestinal conditions and you don't have to have all of them either. If you are really concerned you would be best served by seeking an in-person evaluation.