How to determine the difference between a bad UTI and appendicitis?

Varies. Appendicitis usually continues to get worse over 12-24 hours with worsening right lower abdominal pain, nausea, vomiting, maybe fevers. Appendicitis will continue to get worsening symptoms with more time. Uti may have a lower abdomen pressure discomfort. See a doctor for an evaluation to tell what you may have & what treatment you need.