A dentist or other health care provider can usually identify mouth sores through visual examination of the affected area (e.g., lips, inner cheeks and gums). Most mouth sores (e.g., cold sores, canker sores, or those due to thrush or leukoplakia) can be diagnosed based on the unique appearance and location of the damaged tissue.

A medical history may be taken, including questions regarding current medications, dental hygiene practices and any additional symptoms experienced along with the mouth sores.

Although not usually necessary in otherwise healthy patients, additional tests may be performed. These tests may include:

• X-rays. Radiographic images of the mouth that can reveal the internal structure of the area. X-rays may help identify the extent of infection or tissue damage in patients with mouth sores.
  
  
• Oral brush biopsy. A quick and painless procedure in which a small brush is used to remove some tissue from inside the mouth, which is then analyzed in a laboratory. No anesthesia is required during this type of biopsy. This test helps to identify abnormal cells inside the mouth that may be responsible for a patient’s mouth sores.
  
  
• Biopsy. Removal of a small tissue sample for laboratory analysis. This may sometimes be done in cases of discolored patches of skin (e.g., due to thrush, leukoplakia).
  
  
• Blood tests. Blood is drawn, usually from a vein, and then analyzed in a lab. Blood tests can identify viral infections in patients with mouth sores.