What is the signs of skin cancer to a guy in his 20's?

Same as guy in 50s. Ulceration. Bleeding. Growth, change, variable pigmentation, and more.
ABCD's. Remember your abcd's when thinking of melanoma. Look for moles with asymmetry boarders that are irregular color differences within the same mole diameter greater than 6mm also any new mole or old that starts to itch, bleed or ulcerate should be of concern. Another rule is the ugly duckling rule. Which refers to a mole that looks distantly different from the others on your body.