Diagnosis of Nail Fungus:

Doctor will likely examine your nails first. To test for fungi, your doctor may scrape some debris from under your nail for analysis.
The debris can be examined under a microscope or cultured in a lab to identify what's causing the infection. Other conditions, such as psoriasis, can mimic a fungal infection of the nail. Microorganisms, including yeast and bacteria, also can infect nails. Knowing the cause of your infection helps determine the best course of treatment.