Here is a good guide for when you should be suspicious of moles:

A: Asymemetry: One half of the mole is unlike the other half
B: Border: The border of the mole is irregular. Scalloped or poorly defined
C: Color: The mole color is varied from one area to another. Shades of tan, brown and black - sometimes white red or blue.
D: Diameter: The diameter of the mole is larger than 6 mm - approximately the size of a pencil eraser.
E: Evolving: A mole that changes in size, shape, color, or texture should be checked out. If it tingles, itches, burns, bleeds or oozes - check it out! Also, a sore that does not heal is also a warning sign.