It doesn't matter whether you're 20 or 60 or how many times you've tanned, skin cancer develop in virtually anyone. One of the most common ways to detect skin cancer is through abnormal moles. The following ABCD rule helps tell a normal mole from a melanoma, which is the rarest but most dangerous type of skin cancer:
A: Asymmetry -- one half of the mole does not match the other half.
B: Border irregularity -- the edges of the mole are ragged or notched.
C: Color -- the color of the mole is not the same all over. There may be shades of tan, brown, or black, and sometimes patches of red, blue or white.
D: Diameter -- the mole is wider than about 1/4 inch (although doctors are now finding more melanomas that are smaller).
Other important signs of melanoma include changes in size, shape, or color of a mole. Some melanomas do not fit the descriptions above, and it may be hard to tell if the mole is normal or not, so you should show your doctor anything that you are unsure of.
Source: American Cancer Society