See a dermatologist. 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. Often times, a dermatologist can tell by looking at a spot if it is cancer or not. If you have a spot that bleeds, doesn't heal, or grows rapidly, it may be signs of skin cancer. Fortunately, most are easily treatable in the office.
See your skin doctor. A dermatologist can biopsy anything suspicious to get the diagnosis. Short of that, even experienced skin doctors are wrong as much as 10% of the time.
See your derm. The best way is to have a complete skin examination by your friendly dermatologist.