We can test animals' hearing using a BAER test (https://www.healthline.com/health/baer-brainstem-auditory-evoked-response https://www.healthline.com/health/baer

We can test animals' hearing using a BAER test (https://www.healthline.com/health/baer-brainstem-auditory-evoked-response https://www.healthline.com/health/baer-brainstem-auditory-ev...) - the same one that we use on human infants for early detection of hearing issues. I don't know if it can detect 'sounds' caused by tinnitus though.