Individuals should seek medical advice if a new skin growth appears or an existing mole or skin growth changes and does not heal in two weeks. In most cases, individuals will be referred to a dermatologist, a physician who specializes in conditions of the skin. A medical history and complete physical examination will most likely be the first steps in diagnosing the changes to a mole. To diagnose melanoma, the physician will biopsy the suspicious area by local excision. The sample will then be sent to an experienced pathologist who can view the specimen under a microscope for evidence of cancer. The pathologist can determine the type and stage of the cancer with a biopsy.

An incisional or excisional biopsy is usually recommended to remove suspected melanomas. An incisional biopsy removes a portion of the tumor to be examined later whereas an excisional biopsy removes the entire tumor.

The National Cancer Institute (NCI) recommends that suspected melanomas never be shaved off or cauterized for removal. In addition, NCI recommends that a second review by an independent qualified pathologist who is specialist in skin conditions and diseases be performed to  reduce the possibility of misdiagnosis.

After melanoma is diagnosed, a physician will examine surrounding lymph nodes for signs of enlargement. If the nodes are enlarged, a lymph node dissection may be performed. This procedure involves removing several lymph nodes and examining the tissue for indication that the cancer has spread. An alternative to a complete lymph node dissection is a sentinel node biopsy, which is a procedure done by a surgeon with specialized training. It involves the removal of one or a very limited number of lymph nodes that are most likely to contain cancer cells that have spread. This procedure can predict the likelihood of more extensive lymph node involvement.

The information on lymph node involvement (whether or not they contain any cancer cells) is used for staging the cancer, treatment planning and prognosis. If the melanoma is suspected to have spread, a number of imaging tests may be ordered. These tests include the following:

• Chest x-ray. A painless test in which an image is created of part of the body by using low doses of electromagnetic radiation that are reflected on film paper or fluorescent screens.
  
  
• MRI (magnetic resonance imaging). A powerful magnetic field is used to create images of structures and organs within the body. MRI can be used to help detect several types of cancer, including melanoma that has metastasized to the brain, spinal cord or organs.

• CAT scan (computed axial tomography). A painless test that uses multiple x-ray images, taken from different angles, to create three-dimensional images of body structures.

Sometimes melanoma can develop in the eye. This is referred to as intraocular melanoma, which is a rare form of the disease. Malignant cells develop in a part of the eye called the uvea, which, like the skin, contains melanocytes. This type of melanoma often appears as a dark spot on the iris but can be inside the eye and not visible without special instruments to examine the eye's interior. If the melanoma develops on another part of the uvea, it may cause blurry vision or it may produce no symptoms at all. This form of melanoma is often diagnosed during a routine eye examination and may require an ultrasound for further classification.