what everyone else said above plus:
E1 is a uni-molecular reaction. This means that the reaction occurs in two steps...the first is the formation of a carbocation after the alcohol is protenated by the acid and leaves. Any time a carbocation is formed the reaction is SN1/E1. However, we can determine the reaction goes E1 because the acid catalyzes the dehydration of the molecule (hence the leaving of the water --> dehydration) and forms an alkene. Hope this helps!!