The only way to determine that a mole is cancerous is to consult with a dermatologist. After looking at the mole, the doctor may take a tissue sample to send to a laboratory to confirm or rule out skin cancer.
Signs that suggest a mole may be cancerous include the following:
It is asymmetrical (i.e., not round)
It has an irregular or poorly defined border
It contains many colors, including a mix of tan, brown, red or pink
Its diameter is larger than the size of a pencil eraser
It differs in appearance from other moles, or undergoes changes in size, shape or color