Look at your nails. Healthy nails have a uniform color slightly paler than your skin. They should be smooth and shiny. Look at them straight on by pointing to your eye. A healthy nail bed will be slightly curved into an arc.
WHAT IT MEANS
If your nails are very pale with a dark rim around the edges, this can be a sign of liver disease. The white appearance is a combination of decreased circulation and build-up of connective tissue. If your nails are cracked or peeling, it can be a sign of poor nutrition or thyroid disease. When your thyroid is in overdrive, it increases your nails' growth, not giving them a chance to properly develop and resulting in dried-out nails that crack.
If your nails are indented, or spoon-shaped, iron-deficiency anemia might be the cause. Iron-deficiency anemia is a reduced number of red blood cells because of a lack of iron in the body. Not enough iron in the body is usually a result of poor nutrition.
If your nails are yellow, it could just be stains from nail polish. If it buffs out, it's a stain. If it doesn't buff out, it could indicate the presence of diabetes. Uncontrolled diabetes results in nails that are permanently yellow. The reason for this is that extra blood sugar attaches itself to the nails' collagen, turning it yellow. It could also indicate a fungal infection, especially with the toes.
WHAT TO DO
If you suspect diabetes or thyroid disease, see your doctor.
If you suspect a fungal infection, it can be cleared by anti-fungal tablets.
If you suspect iron-deficiency anemia, increase your intake of iron containing foods and/or begin taking an iron supplement.