Melanoma is a type of skin cancer. Although it is the least common form of skin cancer, it is considered the most aggressive and most dangerous form. It begins in the outermost layer of the skin, the epidermis. In men, melanomas usually occur on the chest and back; in women the most common location is on the legs, neck and face. Melanoma can appear anywhere, although it is less common.
When melanoma spreads to deeper layers of the skin or to other parts of the body, it is called metastatic melanoma, which occurs in stage III or IV melanoma. The cancer most often spreads to the lymph nodes, but can spread to many different areas of the body, including:
Other areas of the skin
When the cancer reaches the lymph node 5 year survival rates decrease. According to the Skin Cancer Foundation, approximately 98 percent of those with melanoma (before it has spread) can be cured if the cancer is detected and treated early. Once it reaches the lymph nodes, the 5 year survival rate decreases to 62 percent and when it spreads to other organs, it further decreases to 15 percent. 
Diagnosing Metastatic Melanoma
If your doctor believes that your melanoma has spread to other parts of your body, he will order any of a number of diagnostic tests, such as:
Blood test for lactate dehydrogenase, which increases when melanoma metastasizes
Imaging studies including: chest x-ray, CT scan, MRI, PET scan or ultrasound
Testing a sample of your lymph node
Fine needle biopsies of potentially affected areas such as breast, thyroid, lung, liver and other locations
You may not experience any symptoms indicating that the cancer is spreading, therefore, it is important to have regular check-ups with your doctor, especially if you have had melanoma in the past. You should also complete self-checks of both your skin and your lymph nodes and contact your doctor if you notice any changes. Catching melanoma early is the best chance to prevent it spreading and giving you the best chance of your treatment being successful.
"Diagnostic Accuracy of Fine Needle Biopsy for Metastatic Melanoma Implications for Patient Management," 2008, January, Anna Doubrovsky et al, Annals of Surgical Oncology
 "Skin Cancer Facts," Updated 2012, Sept 10, Staff Writer, The Skin Cancer Foundation
"Melanoma Skin Cancer," Revised 2012, Sept 20, Staff Writer, American Cancer Society