Hold that polish! Or at least give your nails a good look before you paint them. Your fingernails and toenails are like 20 mini-mirrors to the state of your overall health. Changes throughout the body that are otherwise invisible can sometimes be first seen in the nails, says dermatologist Amy Newburger, a senior attending physician at St. Luke's - Roosevelt Medical Consortium in New York City.

Fingernails tend to give more reliable clues than toenails, given the wear and tear of walking, tight shoes, and slower foot circulation over time, which can obscure toenail changes. But check both hands and feet -- the first of the following nine nail clues explains why.

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Clue 1: A black line

Look for: A black discoloration that's a straight vertical line or streak and grows from the nail bed, usually on a single nail. About 75 percent of cases involve the big toe or the thumb, according to a review in the British Journal of Dermatology. Especially worrisome: a discoloration that's increasing or that's wider at the lower part of the nail than the tip. "That tells you that whatever is producing the pigment is producing more of it," says Newburger.

Also beware when the skin below the nail is deeply pigmented as well, says podiatrist Jane Andersen of Chapel Hill, North Carolina, a spokeswoman for the American Podiatric Medical Association.

What it might mean: Melanoma, the deadliest form of cancer. People with darker skin are more vulnerable than Caucasians to subungual melanoma (melanoma of the nail bed), but darker-skinned races also have more dark lines in nails that are benign, according to a 2004 report in American Family Physician.

Next steps: Always have a doctor check out a suspicious black line on the nail quickly because of the high skin cancer risk. A black line on the nail may also be caused by a harmless mole or an injury. A biopsy can confirm melanoma.

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Clue 2: Small vertical red lines

Look for: Red (or sometimes brownish red) streaks in the nail. "They look like blood or dried blood," Anderson says. These are known as "splinter hemorrhages" because they look like a splinter but are caused by bleeding (hemorrhage) under the fingernail or toenail. They run in the same direction as nail growth.

What it might mean: Heart trouble. The "splinters" are caused by tiny clots that damage the small capillaries beneath the nail. They're associated with an infection of the heart valves known as endocarditis. Don't panic if you see one, though: Sometimes an ordinary injury to the nail can cause a splinter hemorrhage.

Next steps: No treatment is needed for the splinter hemorrhage itself. A doctor can evaluate and treat the underlying cause if it's heart-related.