 [ Symptoms ] 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  [ Disease ] 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