How is skin cancer differentiated from other lesions?

Melanoma: The most important warning sign for melanoma is a new spot on the skin or a spot that is changing in size, shape, or color. Another important sign is a spot that looks different from all of the other spots on your skin (known as the ugly duckling sign). If you have any of these warning signs, have your skin checked by a doctor.

With a microscope: There are many skin lesions that can look like cancer. There are also different types of skin cancer. The only way to distinquish these different types of lesions is to take a biopsy and have a pathologist look at the case under a microscope. The pathologist will then have a report made that will tell your doctor what the lesion is.

Biopsy: All skin problems (benign or otherwise) are differentiated ultimately by their appearance under the microscope. Each skin cancer, rash, etc has a different appearance under the microscope. So...If you have a concerning lesion, it will probably need to be biopsied by your md.