Most moles are entirely benign, but a set of well-established warning signs — commonly taught as the ABCDE rule — are worth knowing and checking for periodically.
The ABCDE warning signs
Asymmetry (one half doesn't match the other), Border irregularity (ragged or blurred edges), Color variation within the same mole, Diameter larger than about 6mm (a pencil eraser), and Evolving (changing in size, shape, or color over time) are the standard signs to check for.
Any single sign warrants a look
A mole doesn't need to show all five signs to be worth a professional check — any one of them, especially a mole that's visibly changing over weeks or months, is reason enough to have it examined.
Regular skin checks matter beyond active red flags
Periodic self-checks (and professional skin exams, especially for higher-risk skin types or family history) catch changes earlier than waiting for an obvious problem — a habit worth building regardless of current risk level.
Does having a lot of moles automatically mean higher risk?
A higher total mole count is associated with somewhat higher skin cancer risk statistically, but it's the behavior of individual moles (per the ABCDE signs) that matters most for an individual check — a dermatologist is the right resource to assess personal risk factors.