In diagnosing thrush, a physician will perform a complete dental examination and compile a thorough medical history. The patient will also likely be asked about any use of antibiotics and medications that suppress the immune system (e.g., chemotherapy drugs, steroids) or about other medications that may be the source of dry mouth.

Thrush has a distinct appearance. A dentist or physician can usually diagnose the condition by looking for the characteristic white lesions on the mouth, cheeks and tongue. When the physician gently brushes away these patches, it will typically reveal tender, reddened areas that may bleed lightly.

Dentists or physicians unable to visually determine whether thrush is present may perform a KOH prep test (a microscopic examination of scrapings from the mouth) to confirm the diagnosis. In some cases, a biopsy will be necessary. In this procedure, a scalpel is used to remove a small piece of tissue that is treated with a special stain and analyzed at a laboratory. 

Patients who have persistent thrush infections may be checked for other illnesses, such as undiagnosed diabetes, cancer or HIV infection. Blood tests or other diagnostic procedures may be performed for this purpose.

Left untreated, thrush can spread into other regions of the body, causing serious complications, such as Candida esophagitis. This condition occurs when thrush extends into the esophagus (which connects the mouth to the stomach).  Diagnosis of Candida esophagitis may require additional tests, including:

• Throat culture. The back of the throat is swabbed with sterile cotton and the micro-organisms are studied under a microscope.
  
  
• Endoscopy of the esophagus, stomach and small intestines. A flexible camera captures images of the lining of the esophagus and surrounding areas.
  
  
• X-rays of the esophagus. An image of the esophagus is produced with low doses of radiation. In some cases, the patient will swallow a chalky substance called barium that will highlight certain organs more clearly on the x-ray.