How can skin problems be diagnosed?

By a doctor, yo'.

A variety of skin tests may be performed to diagnose skin allergies, bacterial or fungal skin infections, and other problems affecting the skin. Skin tests are also performed to tell the difference between malignant (cancerous) cells and benign (noncancerous) growths.

The most common skin tests include:

Patch testing: Patch tests are used to help diagnose skin allergies. Identified allergens (substances that a person may be allergic to) are applied to the skin on the back with adhesive patches and left for a period of time. The skin is then examined for any reaction.
Skin biopsy : Skin biopsies are performed to diagnose skin cancer or benign skin disorders. During a skin biopsy, skin is removed (after a local anesthetic is applied) and is taken to a laboratory for analysis. Skin may be removed with a scalpel, Gillette blue blade, or a cylindrical punch biopsy tool. Stitches may be used to close the wound.
Culture: A culture is a test that is done to identify the microorganism (bacteria, fungus, or virus) that is causing an infection. Skin (surface scrapings, biopsies, contents of pus bumps and blisters), hair, or nails may be cultured to detect bacteria, fungi, or viruses.

Doctors can identify many skin disorders simply by looking at the skin. Revealing characteristics include size, shape, color, and location of the abnormality as well as the presence or absence of other symptoms or signs. To check the distribution of a skin problem, the doctor often asks the person to undress completely, even though the person may have noticed an abnormality on only a small area of skin.
Biopsy:

Sometimes, a biopsy, in which a small piece of skin is removed for examination under a microscope, must be done. For this simple procedure, the doctor generally numbs a small area of skin with a local anesthetic and, using a small knife (scalpel), scissors, razor blade (called a shave biopsy), or round cutter (called a punch biopsy), removes a piece of skin the size of which is determined by the type of lesion, its location, and the type of tests to be done.
Scrapings:

If a fungal infection or scabies (see Scabies) is suspected, a doctor may do a skin scraping. During this test, the doctor scrapes off some material from the skin, such as scale, and examines it under a microscope. Sometimes special chemicals or stains are applied to the material.
Culture:

If an infection is suspected, a sample of material (such as a skin scraping) can be sent to a laboratory, where the specimen is placed in a culture medium (a substance that allows microorganisms to grow). If the specimen contains bacteria, fungi, or viruses, they will often grow in the culture and can then be identified.
Wood light:

A Wood light examination is used when certain skin infections are suspected. The skin is illuminated with an ultraviolet light (also known as a black light) in a dark room. The ultraviolet light makes some fungi or bacteria glow brightly. The light also accentuates the skin's pigment (melanin), making pigmentation abnormalities, such as vitiligo (see Vitiligo), more visible.
Tzanck testing:

A Tzanck test is done to help a doctor diagnose certain diseases caused by viruses, such as herpes simplex and herpes zoster. When these diseases are active, they produce small blisters. During a Tzanck test, a doctor removes the top of a blister with a sharp blade and then scrapes the blister with a scalpel to obtain fluid. The specimen is examined with a microscope after special stains are applied.
Diascopy:

Diascopy is done so that a doctor can observe color changes that occur when pressure is applied to the skin. During this test, a doctor presses a microscope slide against a lesion to see whether it blanches (whitens) or otherwise changes color. Certain kinds of lesions whiten, whereas others do not. Some skin lesions (such as those caused by sarcoidosis—see Sarcoidosis) turn an apple jelly color when this test is done.

Thanks for information sharing about skin problems and how it can be diagnosed.