Tests can be done on blood, stool, urine, saliva, and tissue or fluid samples.
Blood tests will check for antibodies to candida - IgA, IgG, IgM, or by using PCR testing to check for the DNA of candida. PCR testing is highly sensitive. Blood can also be cultuted to see if candida is growing and present.
Stool testing can check for cultures or use the PCR testing to check for DNA.
Saliva can test for cultures and visualization via microscopic examination. Note: the saliva test where someone spits in a glass and looks for threads of mucous hanging down is not a test for candida. It was marketed that way. Mucous will thicken and create hanging threads with virus, bacteria, yeast, mold, fungus, allergies, chemicals, heavy metals, etc.
Urine tests check for metabolites and waste products of candida.
Direct tissue or fluid samples can culture and check via microscopic exam.
Since these tests can produce false positives and negatives, it's best to use an approach which considers several factors - symptoms; history of antibiotic use; lifestyle factors; testing; and response to treatments.
From the research that I've analyzed, history of antibiotic use is a positive indicator.