How can you tell the difference between impetego and a cold sore? thanks

Usually by: Its appearance, but need cultures to make sure. Treatment is different, so it is kind of important to see a doctor, who will be able to tell you which it is. Impetigo is caused by bacteria and cold sores by a virus. Cultures take few days to come back, and results might be too late for effective treatment.

Easy for a dermatolo: These are very different. A dermatologist will know immediatly. Impetigo is contagious, painful, spreads, shows honey-colored crusts, responds to antibiotic. Cold sore, unless an initial infection (rare), is localized, limited, returns to the same place. Has a very different appearance.