Obviously, not responding when their name is called would be a pretty big indicator; similarly, not showing interest when someone makes a sound nearby but out of their sight may indicate hearing loss. However, autistic children are sometimes mistaken for deaf because they may not respond if they are focused on something else or don't understand that you want them to respond when you say their name, or may not turn around to investigate a sound even though they can hear it.
There are a couple of tests that a doctor can do to test hearing in a child who does not have the ability to communicate yet--ABR (auditory brainstem response) or OAE (otoacoustic emission) tests. These are noninvasive screenings often used on newborns which basically test whether sound can get from the ear to the brain. They can detect many different types of hearing problems, though not auditory processing disorder (auditory processing is the process of extracting useful meaning from a sound you have heard; APD is a type of sensory integration disorder and involves difficulty interpreting sounds in your environment.)
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