How can the dr tell the difference between just fluid build up in an ear after a cold and an actual ear infection?

Looking: A physician or other care provider can determine the difference between fluid build-up(serous otitis) and an actual infection by examination of the ears with an otoscope--serous fluid id clear/orange and may have air bubbles, an infection will be yellow/white, the eardrum will be opaque, dark red or distorted. Fever and pain are not adequate to differentiate the two.