Hello and welcome. I am Dr. B and would like to help you today.I'm regards ***** ***** question, the way a vet diagnoses ear mites or bacterial/yeast infections
is by examining the ear and the discharge present.
Ear infections tend to inflammation with snotty discharge, where mites cause brown discharge or sometimes just inflammation. As well, to make it a bit more challenging, if yeast is present, this too can cause brown discharge but often has a bread type smell.
Since these findings can all be a bit similar and we often see mixed infections of more than one, we often will take a discharge sample at the time of exam. Ear mites can be checked for under the microscope; where the others can be tested by the vet lab to see which species are present and what drugs they are sensitive to.
So that is how we tell the difference. Though if you susepct mites, do note that multi-species anti-parasitic medications (ie advantage multi, revolution, etc) can be used to help clear them. As well, for mild infections you can flush the ears with salt water to remove discharge and reduce bacterial levels present. Otherwise, if her ear is really bothering her, we'd want to have her local vet carry out the above to diagnose what is present here so the appropriate drops can be dispensed to address this for her.