How do i know if i have a fever blister or a cold sore?

One and same thing: Fever blisters and cold sores are the same names for painful ulcerating blisters caused by herpes simplex virus. Many people carry the virus, it "hides" in the nerve trunks that supply the face. Usually occur on outside border of lower lip & rarely inside the mouth as well. It starts as a tingling sensation and becomes painful with time. Stress is a major trigger. It will go away~2 weeks.