How can you tell the difference between herpes and a kanker sore at your lip area?

Culture & DFA. To clinch the diagnosis of herpes simplex and rule out a simple canker sore, your doctor can perform a skin swab test to culture the herpes virus and or send a skin swab for a very specific test call a DFA (direct fluorescent antibody test). The DFA is very accurate and will improve the diagnostic accuracy. However, the DFA is a fairly expensive test and a culture should usually suffice.