(MONEY Magazine) – You just turned 40, or maybe 45. And while you don't exactly feel different, you've begun to sense that on matters of health, it's time to keep your eye on the ball in a whole new way.

Yes, your forehand still sizzles and you can do almost all you want physically, but that old confidence in your immortality seems to have wilted sometime between your 20th and 25th college reunions. Now this is not another scolding to eat broccoli, run three miles a day and get seven hours of sleep every night, although of course you should. Instead, this story is about the decisions you'll have to make on a health-care issue where money and medicine increasingly clash: how much capital (both monetary and mental) you should spend on medical tests to track the state of your health. On pages 116 to 119 we outline a baseline battery of tests that pass all reasonable measures of cost-effectiveness and often make sense for both men and women. On page 120, veteran health writer Curtis Pesmen highlights five unconventional screenings that he believes are worth paying for even if your insurer doesn't approve. But wait, you may be thinking, shouldn't I simply let my doctor decide which tests I need? Well, that's one strategy. But your particular doc might not be prescribing the tests you really need. The Journal of General Internal Medicine reports that many annual physicals "include tests of unproven value while several recommended tests are often not done." You can demand more than what's recommended, as do many women who insist on annual Pap smears when evidence suggests getting one every three years is sufficient. But that's not ideal either. Every test has costs--and not only in money. Many are invasive and carry health risks of their own. Others are prone to false positives (such as mistaking a cyst for cancer) that can provoke anxiety and further costly tests and treatments. "Any kind of screening is a trade-off," says Dr. Susan Drossman, a New York City radiologist. "You need to ask: How much insight will you gain for the money you spend and the risks you take?"

Guidelines You Can Trust

Just as financial minds disagree on the best way to boost your return while lowering the risks in your portfolio, medical minds diverge when it comes to balancing the risks and benefits of many screening tests. It's not just individual doctors who disagree; different medical disciplines and advocacy groups may have conflicting priorities or approaches. The American Cancer Society, for example, recommends that doctors offer to screen men for prostate cancer starting at age 50. The National Cancer Institute, however, says there's no hard evidence that this practice ultimately saves lives.

Skeptics say you should also consider the economic self-interest of whoever is recommending a test. The American Academy of Dermatology calls for annual skin-cancer screenings by a specialist, yet the American Cancer Society says a general practitioner should suffice.

So whose cost-benefit analysis can you trust? The gold standard of screening guidance is the U.S. Preventive Services Task Force, which was created in 1984 by the U.S. Public Health Service and charged with producing unbiased recommendations from the best and latest research. The USPSTF (website: www.ahrq.gov/clinic/uspstfix.htm) is composed of 16 physicians and other medical experts who hold monthly conference calls and meet in person three times a year. One goal is to figure out the net benefit--that's benefit minus harm--of each particular test. They look not just at lives saved but also at the risk of false positives, too many tests or anxiety over the test itself. "We take advocacy and passion out of the process and rely on cold, hard facts," says Dr. Ned Calonge, Colorado's chief medical officer and current chairman of the USPSTF.

The next four pages map out tests the task force recommends most highly for men and for women. All the tests described as "must have" earned a USPSTF rating of A (the task force "strongly recommends" it) or B ("recommends" it). Tests described as "optional" are recommended by the dozens of doctors and medical experts we contacted for this story.

Getting Your Care Paid For

The good news is that when the task force issues an A or B rating, the insurance industry tends to listen--and insurers are more likely to approve preventive care than they once were. But they also watch what Medicare and the states require, which can vary widely. California does not make insurers cover colonoscopies; Virginia does, thanks largely to the late state senator Emily Couric (sister of TV's Katie, whose husband died of colon cancer).

If your insurer doesn't cover a test, you have a decision to make. Ask your doctor the questions below, and if you still want the screening, consider fighting for reimbursement. Every insurer (including Medicare) has an appeals process. "If you're really assertive, you'll win more often than you'll lose," says Peter George, a consultant who heads Healthcare Litigation Support in Petersham, Mass. But you must do enough medical research to know both sides of the argument, he warns, so that you can make a solid case for being an exception to their rules. "One way to succeed," he says, "is to show them that if you don't get the test done, it may cost the insurance company more through expensive treatments later on."

Whatever tests you have done, get a copy of your records on paper or DVD so the information is handy if you move or visit a new specialist. Most doctors are happy to comply; if not, offer to pay the duplicating costs. You're at an age now at which you need to take ownership not just of your diagnostics but also of the data from those tests. You're in your prime, after all. And you want to stay that way.