How to tell the difference between a burn blister and a fever blister?

Blisters. A burn blister is usually one blister and a fever blister (herpes simplex virus infection or reactivation) is usually multiple blisters grouped together, although that is not an iron clad rule. In immunosuppressed patients herpes blisters can be solitary blisters. Also, if you burn you lip, the trauma of the burn can cause the herpes blisters to appear if you have been infected with hsv.
Blister. Your dermatologist would be able to look at it and tell. If there is uncertainty the blister fluid could be examined/cultured for evidence of herpes virus.