WHAT TO LOOK FOR: A crusty material accumulating along the eyelash line. Your eye might also feel irritated, gritty or itchy. Up to half of all adults will develop this condition, called blepharitis. The issue is easy to self-diagnose, says Olivia Killeen, M.D., an ophthalmologist at the University of Michigan.
HOW TO DO IT: Mix a few drops of tearless baby shampoo, gentle facial cleanser or commercial eyelid cleansing solution with a bit of water until frothy, then gently massage it into one eyelid at a time, rinsing well. If the flakes dissolve and the irritation subsides, you’ve got this under control; simply follow this routine every few days.
WHOM TO CALL: If this simple treatment doesn’t pan out, you may have a more serious issue. Contact an optometrist or ophthalmologist.

WHAT TO LOOK FOR: Plaque, a sticky bacterial film that contributes to gum disease.
HOW TO DO IT: Purchase some disclosing tablets—chewable tablets that temporarily stain plaque a bright color like pink or blue. After chewing, look for any colored spots; use a little handheld mouth mirror to see the back of the teeth.
WHOM TO CALL: Your dentist. Early-stage gum disease is usually reversible with a professional cleaning and an upgraded oral hygiene routine; late-stage gum disease, called periodontitis, is not, says Lauren Becker, D.D.S., a general and cosmetic dentist in New York City.

WHAT TO LOOK FOR: Lumps or bulges while swallowing. These could signal thyroid trouble, including hypothyroidism (an underactive thyroid), hyperthyroidism (an overactive thyroid), thyroid cancer or a goiter (an enlarged thyroid).
HOW TO DO IT: Have a glass of water nearby. Using a handheld mirror, zero in on the lower front area of your neck. Take a sip of water, tilt your head back slightly (keeping your eyes on the mirror) and swallow, looking for any bulges or lumps that appear in your neck. Don’t confuse your Adam’s apple for your thyroid; the thyroid sits below the Adam’s apple, closer to the collarbone, and is generally not visible unless something is amiss.
WHOM TO CALL: Your family physician, internist or endocrinologist, who will examine your neck and order any necessary tests.

WHAT TO LOOK FOR: Changes in size, shape or skin texture—possible warning signs of breast cancer or a fibroadenoma (noncancerous breast lump).
HOW TO DO IT: Standing topless, place your hands on your hips, gently pressing them in to flex your chest muscles. (Men: You need to do this too.) Look for any changes, including lumps, dimpling or puckering, new asymmetry between the breasts, a newly inverted nipple, redness or rash, or leaking fluid. Next repeat the sequence with your hands raised above your head. This helps you “see the side of the breast and armpit better,” says Karen L. Smith, M.D., a family physician in Raeford, North Carolina. WHOM TO CALL: Your obgyn, family physician or internist, who will perform a breast exam and order any necessary tests.

WHAT TO LOOK FOR: New spots or lesions, or preexisting moles that seem to be changing color, shape or size. Women should pay special attention to their lower legs, and men, to their backs.
HOW TO DO IT: Closely inspect your face and ears, torso (including beneath the breasts), arms and underarms, and the fronts of your legs. Now turn your back to the mirror and snap a photo over your shoulder with your phone; use the zoom function to scan your back.
WHOM TO CALL: Your dermatologist, who will thoroughly examine your skin to identify any potentially cancerous lesions.

WHAT TO LOOK FOR: A hunched-over posture that resembles a Cinstead of an S, a sign of tight or weak muscles or possible vertebral fractures.
HOW TO DO IT: Stand sideways before a full-length mirror, legs hipwidth apart and the crown of your head lifted toward the sky. Turn your head toward the mirror. Your ear, shoulder, hip, knee and ankle should all be in alignment, with a gentle S shape to your back (curving slightly outward behind the lungs and inward near the low back). If your back is more C-shaped, with your head pushed forward, or has a hump, try to stand up taller. If you can’t, you may have vertebral fractures due to osteoporosis, says Carol Krucoff, codirector of Integrative Yoga for Seniors teacher training at Duke Integrative Medicine in Durham, North Carolina.
WHOM TO CALL: Your health care provider, who may recommend yoga or physical therapy. If they suspect osteoporosis, they might refer you to an endocrinologist for a bone density scan.

WHAT TO LOOK FOR: Unexplained thinning or patchy hair loss.
HOW TO DO IT: Run a hand through your hair and look to see if strands come out. If they do, use a magnifying glass to examine the ends of the strands. If they are frayed, that probably indicates damage from overuse of a hair dryer or dyeing. If the ends are tapered, it means otherwise healthy new hair is coming out, a phenomenon that can occur several weeks to months after an intense period of stress or illness, says Sara Wasserbauer, M.D., a past president of the American Board of Hair Restoration Surgery.
WHOM TO CALL: Talk to your family physician, internist or dermatologist, who may recommend topical or oral hair regrowth medications.

WHAT TO LOOK FOR: Changes in your arches when you shift your weight.
HOW TO DO IT: Stand barefoot in front of a full-length mirror and turn sideways so your right side is closest to the mirror. Shift your weight onto your right foot, slide your left foot forwards several inches and look at the arch of your left foot. Now shift your weight onto the left foot. If your arch falls flat, “it could indicate that the tendon that holds up the arch may be weakening,” says Marlene Reid, a surgeon at Family Podiatry Center in Naperville, Illinois. This often occurs in just one foot, so repeat the process on the other side.
WHOM TO CALL: Your podiatrist, who may prescribe orthotic inserts and recommend arch-supportive shoes if you’re having discomfort.

Science journalist Leslie Goldman holds a master’s degree in public health.