the only definite 100% way of determining if a skin lesion is a skin cancer or not is with a biopsy. either the entire lesion ( or a piece of the lesion) is removed and analyzed by a pathology lab. a skilled clinician like a dermatologist can give you a pretty-good idea but a visual inspection can be wrong. because of your family history of melanoma it would be wise to get it checked out.
how do you know a purple spot is cancer? . the spot is lightly raised and is on the inside of my right forearm. i also have a spot (raised that appeared on the forehead but looks like a blister. it is getting dryish but still raised. my brother had melanoma my grandmother had a type of stomach cancer and my dad had some type of cancer but not sure what kind. .