How could a doctor tell the difference between a kidney infection and a uti?

Definitions: First of all, both of these are very generic terms. Uti refers to infections anywhere along the urinary tract including a kidney infection. However when most people use the term UTI they usually mean a bladder infection or irritation of the lower urinary tract. Infections of the kidney or pyelonephritis are often associated with fevers, severe pain and more serious illness than bladder infection.