A diagnosis of oral thrush is made by a doctor or medical professional. The doctor will examine the patient's mouth to look for the white patches, especially on the tongue and cheeks. By brushing the patches away, inflammation will be visible. Examination of tissue under a microsocope is a way to confirm the oral thrush diagnosis.

In patients with no known risk factors for oral thrush, a doctor may recommend a physical exam and blood tests in order to determine whether there is an underlying medical issue causing the infection.

If the oral thrush has reached into or beyond the esophagus, additional tests may be necessary for it to be diagnosed. A throat culture, an endoscopy or an x-ray combined with swallowing barium will be the likely choice.