How do doctors tell a difference between a cold sore and a herpes blister?

They are the same!: You can look at the pictures posted her about the variuos forms of herpes i and ii as well as other on-line searches and compare what you have but the only way to be sure is to have a professional evaluated it. Both hsv i & ii sores will look different at different phases in the sore from onset to resolution & a professional will know all the possible appreances.