It depends a bit on what type of mole and where it is. For brown/black moles, the key things to remember are ABCD. A for Asymmetry — a mole that doesn’t have the same shape on both sides, B for Border — a mole that doesn’t have a clear smooth border with the normal tissue around it, C for Color — a mole that has two or more different shades of brown, blue or black, and D for Diameter — a mole that larger than 6 mm or the eraser of a pencil. For skin color or light moles, if it is changing in size, gets irritated or itchy, forms an ulcer or is painful, I would have my family doctor look at it.