To help you recognize the characteristics of unusual (pre-cancerous and cancerous) moles that may indicate the development of melanomas, think of the letters ABCDE: Where
A is for asymmetrical shape
A normal mole will always have a regular round to oval shape. Changes during the cancerous stages can make a mole a bit asymmetrical. You should look for a mole with an irregular shape, such as having a sharp edge, or two or more very different-looking halves.