Various diagnostic tests are performed to detect skin problems like infections and allergies, but common tests include: Patch test - To diagnose skin allergies, allergens are applied on the skin surface, and the skin is examined for any reaction. Culture - The surface of the skin or nail is scraped and tested for the presence of bacteria, virus, or fungus. Biopsy - A sample of the skin is taken from abnormal skin growth or lesion to diagnose conditions like skin cancer.