How do doctors tests infants' hearing?

OAEs, BAERs. Oaes (otoacoustic emissions) testing measures something called "the outer hair cell function of the inner ear." the tool sends sounds in and measures what sort of bounces back. It is not invasive (doesn't hurt) and can be done regardless of age. Baer (brainstem auditory evoked responses) is another test & measures electrical activity. Both are good screens, though neither is perfect.
Newborn screening. Nearly 97% of babies born in the U.S. Undergo hearing screening in the 1st month of life. Special equipment now makes this easy and reliable. It allows doctors to provide important early intervention (hearing aids) to hearing-impaired babies, avoiding speech and language complications.