The most dangerous skin cancer is melanoma. Prognosis is highly dependent on early detection. The most important rule for the detection of melanoma is so called the “ABCDE” rule: A- asymmetry, B- borders (irregular), C- colour (variegated), D- diameter above 6 mm, E- evolving over time.
A good advice is always to look for spots that:
Change size, colour or shape
Look different compared to the rest
Feel rough or scaly; sometimes you can feel the lesions before you can see them
Are bleeding or oozing
Look like a wound but do not heal