How do you know if a mole is cancerous?

Biopsy: A new mole, or one that enlarges or changes appearance needs to be biopsied. That is the way to determine if it is cancerous.

ABCD: I agree that biopsy is the only sure way to tell what it is. Signs to look for on skin lesions are asymmetry, border irregularity, color unevenness, diameter >6 mm.

Biopsy: Is the definitive diagnosis. Dermoscopy is getting a lot of press of being pretty effective at well. This is a non surgical way of looking at the mole thru a magnifying lens with a light. Studies show a very high accuracy rate. (not all docs use one though.).