A helpful guide for assessing moles is the ABCDE rule. A stands for Asymmetry (one half unlike the other); B for Border (irregular, blurred or jagged edges); C for Colour (uneven, with shades of brown, black, red, white or blue); D for Diameter (larger than about 6 mm, the size of a pencil eraser, though smaller melanomas occur); and E for Evolving (any change in size, shape, colour, or new symptoms like itching, bleeding or crusting).
Any mole that looks clearly different from your others (the 'ugly duckling' sign), a new pigmented spot in adulthood, or a sore that does not heal also deserves attention. Most moles are harmless, but early detection of skin cancer greatly improves outcomes.
See a doctor promptly for any mole or skin lesion that is changing, new and unusual, bleeding, or that worries you, rather than waiting to see if it settles. People with many moles, fair skin or a family history should be especially vigilant and consider regular skin checks. This is general information, not a diagnosis.