How is skin cancer usually diagnosed?

By a dermatologist. Spots that don't heal, bleeds, or grows rapidly may be signs of skin cancer. The best way to diagnose a skin cancer is to see your dermatologist. He or she will examine the spot and determine if a skin biopsy is necessary. Most of the time, a dermatologist can tell by looking at a spot if it is cancer or not.
Biopsy. When you have a mole or lesion that your dermatologist is suspicious for skin cancer, he/she will perform a skin biopsy for definitive diagnosis. It is a quick procedure done under local anesthesia.