The commonest method of detection of diphtheria carriers is:
Schick test, method for determining susceptibility to diphtheria; it laid the basis for inoculation against the disease. A minute amount of diphtheria toxin is injected into the skin of the forearm. 
Redness at the site of injection after three days indicates a positive reaction (absence of circulating antibody) or a false positive reaction (hypersensitivity to the toxin). 
A positive reaction can be distinguished by use of a control injection of the same amount of heated toxin (toxoid) into the other forearm.
Schick test
Dick test
Casoni's test
Charles' test