You’ve probably learned by now that some moles can be cancerous. What can be harder to figure out is what to watch for if you have moles and when to visit your doctor.

The American Cancer Society (ACS) suggests using the “ABCD rule” to help distinguish a normal looking mole from one that could be skin cancer. The letters can help you remember what to check for:

A = asymmetry. This means that if you were to slice a problem mole down the middle, one half would not match the other. Normal moles are fairly symmetrical.

B = border irregularity. A normal mole has a fairly regular round or oval border. An abnormal mole may have notched, ragged or indistinct borders.

C = color that’s uneven. Normal moles are usually tan, brown or black with pretty consistent coloring. Moles to worry about are often not the same color all over, and may include patches of black, pink, white or even blue.

D = diameter larger than 6mm. Most normal moles are no more than 6mm or about a quarter inch across, about the size of a pencil eraser. Cancerous moles are often (but not always) larger.

In addition to recommending this easy detection scheme, the ACS also urges you to see a doctor if one of your moles undergoes a change, such as becoming larger, if you see new lesions or growths appear and if you have a sore that doesn’t heal.

It can be difficult to get yourself in the habit of checking for skin cancer. It’s easy to feel weary of being on the alert for breast cancer, colon cancer and all the other things that can go wrong--especially in these media-dominated times when we fear every fever may be swine flu and every case of diarrhea may be the dreaded e. coli. And it’s human nature to want to shield ourselves from potential bad news.

But there are many good reasons to train yourself to monitor your moles. First, skin cancer is very common. In fact the ACS reports that skin cancer accounts for about half of all cases of cancer in the U.S. On the other hand, the vast majority of skin cancers are one of two forms of carcinoma, much less deadly than melanoma, the third possibility.

Although it accounts for less than 5% of all skin cancers, melanoma causes a large majority of skin cancer deaths. What’s the best defense? You got it, early detection.

So go ahead, retrain yourself. Examine your skin monthly from head to toe, literally. Don’t forget your scalp and the bottoms of your feet. You already committed to monthly breast self-exams, you can check your skin at the same time. Think of it as one more way to care for yourself.