A health care provider diagnoses yeast infection based on the findings of a physical examination and seeing yeast from a swab under the microscope. Sending cultures out to a laboratory is sometimes (but not usually) necessary.

In women, redness of the external and internal genital area is a sign of yeast infection. The acidity of the secretions in the vagina is normal (in the other two common vaginal infections, trichomoniasis and bacterial vaginosis, the pH is high). A swab of the vaginal wall will usually reveal yeast under the microscope, although if a woman has self-treated with intravaginal yeast creams within the last few days, no yeast may be seen. If the diagnosis is in doubt and the swab test does not resolve the matter, then a culture may be performed to look for yeast. The Pap smear may reveal yeast, but it is not a sensitive screen and should not be relied upon for diagnosis of yeast infection.

Because treatments for yeast infections are now available over the counter (without a prescription), many women self-treat what they believe to be a yeast infection without seeing a health care provider. Under these circumstances, a woman whose symptoms are due to another genital infection with similar symptoms, such as herpes, may not be properly diagnosed and treated. If she has herpes, she may incorrectly believe that the antifungal medication is responsible for the resolution of the symptoms, when what really happened is that the herpes outbreak resolved on its own. Any woman experiencing symptoms of a yeast infection for the first time, and especially a woman who frequently has symptoms of yeast infection and has never been diagnosed, should see a health care provider for a definitive diagnosis. She should be examined when the symptoms are present, because when they have almost cleared up, or after treatment with creams, it may be difficult to determine the cause of the infection. For men, rubbing the lesions with a cotton swab dipped in normal saline will often show yeast under the microscope, although it is generally more difficult to detect yeast in men than in women by looking under the microscope. If the rash is typical of yeast infection, and other possibilities (such as herpes) have been excluded, sometimes a trial of yeast cream is used as both a therapeutic and a diagnostic tool—that is, if the yeast cream clears up the lesions, then the infection was probably due to yeast.

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