Doctors say to beware of the ABCDs if you are looking for possible problematic moles. They are:
Asymmetry: If the mole is not symmetrical, there could be a higher risk of cancer
Border: Make sure moles have regular borders. If part of the mole is zig-zagged, or crooked, it should be checked.
Color: There should be no purple, blues, greens, red, or whites in any of the mole
Diameter: A mole should be checked if its diameter is larger than the head of a pencil eraser.
Hope this helped!