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The purpose of some of the tests is to rule out other possible causes of the symptoms. For example, the physical and neurologic examinations are to confirm that clinical signs are limited to the masticatory muscles. Polymyositis is a more generalized muscle inflammation of the masticatory muscles and other muscles but otherwise is difficult to distinguish from masticatory myositis. Other possible causes of such symptoms include temporomandibular joint disorders, and endocrine disorders, such as Cushing's syndrome and hypothyroidism.

General anesthesia may be called for to get the dog to open its mouth to check for other possible causes of the pain, such as broken teeth or bones or a dislocated jaw. In advanced chronic cases, the dog’s jaws may not open even under anesthesia.

In a, veterinary researchers reported that magnetic resonance imaging (MRI) scanning detected "widespread, symmetrical, and inhomogeneously hyperintense areas in the masticatory muscle", enabling early diagnosis and therapeutic treatment."

If the dog’s jaw or temporal muscles have started to atrophy (a symptom in the chronic stage of MMM), the examining veterinarian should find out if the dog has been treated with lengthy (seven days or longer) corticosteroid therapy for another disorder, since corticosteroids have been known to cause these muscles to atrophy.

The owner should tell the veterinarian about any recent vaccinations of the dog. Dr. Diane Shelton, board certified in veterinary internal medicine, recently reported that several young cavaliers developed MMM within ten days of vaccinations.

It is important to begin therapy of the MMM-affected dog as early as possible, so that the acute stage does not progress to the chronic stage. However, treatment should not begin before thorough testing and diagnosis.