How can you tell the difference between a canker sore and a fever blister?

Please note that I am not a physician and cannot diagnose nor treat any medical condition.

The quick answer to your question is that canker sores generally occur inside the mouth whereas fever blisters occur in the skin around the lips.

A longer explanation is that fever blisters are usually associated with a viral infection such as herpes whereas canker sores are not.

Fever blisters are also called cold sores. They recurrently erupt as red, painful, raised sores around the lips. Usually they are preceded by a prodrome such as tingling prior to appearing. Treatment for cold sores includes topical antiviral medications, oral antiviral medications, and lysine supplementation.

Canker sores are also called aphthous ulcers. They are quite painful and appear in the gums. They are actually ulcers, meaning that they are open sores. They may be caused by minor abrasion or trauma to the gums, food sensitivites, espeically with acidic citrus fruits, vitamin deficiencies (B12, folic acid, zinc or iron), Bechet's Disease, radiation treatments, ulcerative colitis, Crohn's Disease, stress, a compromised immune system, or using dental products containing sodium lauryl sulfate.

Both types of sores generally resolve on their own. Canker sores may require medical treatment if they are particularly severe.