Biopsy, removing a small piece of skin to examine under a microscope. Culture, taking a skin sample to test for bacteria, fungus or viruses. Skin Patch Test applying small amounts of substances to test for allergic reactions. Black light examination (Wood light test), using an ultraviolet (UV) light to view your skin’s pigment more clearly. Tzanck test, examining the fluid from a blister to check for herpes simplex. Diascopy, pressing a microscope slide against a skin patch to see if the skin changes color. Dermoscopy, using a hand-held device called a dermatoscope to diagnose skin lesions.