This is a medical question of type : exams and tests 
 How to diagnose Skin Cancer ? 
 What Happens During Screening? Checking for cancer in a person who does not have any symptoms is called screening. Screening can help diagnose skin problems before they have a chance to become cancerous. A doctor, usually a dermatologist, screens for skin cancer by performing a total-body skin examination. During a skin exam, the dermatologist or other health care professional looks for changes in the skin that could be skin cancer, and checks moles, birth marks, or pigmentation for the ABCD signs of melanoma. He or she is looking for abnormal size, color, shape, or texture of moles, and irregular patches of skin. Screening examinations are very likely to detect large numbers of benign skin conditions, which are very common in older people. Even experienced doctors have difficulty distinguishing between benign skin irregularities and early carcinomas or melanomas. To reduce the possibility of misdiagnosis, you might want to get a second opinion from another health professional. Self-Examinations You can also perform self-examinations to check for early signs of melanoma. Make sure to have someone else check your back and other hard to see areas. Do not attempt to shave off or cauterize (destroy with heat) any suspicious areas of skin. Risk Tool The National Cancer Institute developed a Melanoma Risk Tool which can help patients and their doctors determine their risk. The tool can be found at http://www.cancer.gov/melanomarisktool/. Performing a Biopsy In order to diagnose whether or not there is skin cancer, a mole or small piece of abnormal skin is usually removed. Then, a doctor will study the suspicious cells under a microscope or perform other tests on the skin sample. This procedure is called a biopsy. It is the only sure way to diagnose skin cancer. You may have the biopsy in a doctor's office or as an outpatient in a clinic or hospital. Where it is done depends on the size and place of the abnormal area on your skin. You may have local anesthesia, which means that you can be awake for the procedure. If Cancer Is Found If the biopsy shows you have cancer, tests might be done to find out if cancer cells have spread within the skin or to other parts of the body. Often the cancer cells spread to nearby tissues and then to the lymph nodes. Has the Cancer Spread? Lymph nodes are an important part of the body's immune system. Lymph nodes are masses of lymphatic tissue surrounded by connective tissue. Lymph nodes play a role in immune defense by filtering lymphatic fluid and storing white blood cells. Often in the case of melanomas, a surgeon performs a lymph node test by injecting either a radioactive substance or a blue dye (or both) near the skin tumor. Next, the surgeon uses a scanner to find the lymph nodes containing the radioactive substance or stained with the dye. The surgeon might then remove the nodes to check for the presence of cancer cells. If the doctor suspects that the tumor may have spread, the doctor might also use computed axial tomography (CAT scan or CT scan), or magnetic resonance imaging (MRI) to try to locate tumors in other parts of the body.