(FORTUNE Magazine) – When it comes to sizing up our own job performance, most of us think we're from Lake Woebegone, Garrison Keillor's mythical town where "all the children are above average." But guess what, kids: If you go around blissfully assuming such a thing, you could very well be in for a nasty surprise one day soon. Rob Norton, a human resources vice president at pharmaceutical giant Pfizer, calls it "that awful shocker of a Friday afternoon"--the moment when, totally out of the blue, you're told that your services are, um, no longer deemed essential.

This has been happening a lot lately. Says Bob Kaplan, a founder of management consulting firm Kaplan DeVries of Greensboro, North Carolina: "It's like a marriage. You go along without really seeing your impact on the other person, and then one day your spouse says, 'I want a divorce.' And you're in absolute shock." Here's the intriguing part: The higher up you are on the organization chart, the more likely you are to be totally clueless until it's too late to do anything about saving yourself.

The answer, of course, is to make sure you have a clear picture of your true value to the organization. And figuring that out is, like so much these days, largely up to you. Consider for a moment what 150 CEOs revealed when asked, in a survey by Philadelphia-based executive training company Manchester Partners, why they had recently canned senior managers--execs who had been just one rung, or at most two, below the chief executive level. In many cases, the CEOs said, these firees were lacking in skills like team building and strategic thinking. But 77% of the CEOs surveyed also confided that the newly departed were in desperate need of an impartial, no-holds-barred performance evaluation.

It shouldn't be terribly surprising that they never got one from the very top dogs who eventually fired them. Says Molly Shepard, Manchester's president: "Most senior people just don't have the skills, or the desire, to convey bad news." Joseph Gibbons, a consultant at Towers Perrin who has put together 360-degree evaluation programs for clients like AT&T, Union Carbide, and Citibank, explains it this way: "When you get on the 'high potential' list at a lot of companies, you have your last formal performance evaluation as a middle manager. Above that, it's all a matter of relationships--we're buddies."

Even if you haven't yet reached that rarefied level, where the CEO is just too close a pal to tell you the truth about yourself, you may be on your own when it comes to an honest evaluation. If you're relying on some formal system of evaluations to tell you how you're really doing, you're probably taking a big risk. At most companies your actual standing is seldom revealed through formal channels.

But are you up to a rigorous self-assessment? Says Manchester's Molly Shepard: "Unfortunately, because we all have a need to cling to the familiar and safe, most people won't go looking for bad news--even if it could save them." She offers four sure-fire tip-offs that your personal stock is sinking fast: (1) You've stopped being invited to important meetings; (2) Your boss suddenly won't make eye contact with you; (3) Your peers know more than you do about what's going on; (4) Responsibilities are taken away from you under the guise of repositioning the business. This last pretext, Shepard says, turns out to be "b.s. 99% of the time."

Some observers, like New York City executive coach Elizabeth Denton, point out that seeing yourself clearly is always hard because "we only see ourselves through the filter of our own perceptions." But in times of rapid change, right now for instance, even a crystal-clear view of your strengths and weaknesses won't help if your company is changing faster than you are. "For many of us, the old standards and values--loyalty, long experience, a network of friends, a sense of history-- are simply not wanted anymore," notes Tod White, an organizational psychologist in Princeton, New Jersey. "If you have a boss who's trying to change the mission and culture of the company, actively supporting that boss will help you get ahead--almost independent of your skills in other areas. Resisters, again regardless of how 'good' they are, will get left behind."

Is this a polite way of saying that a little politically motivated bootlicking goes a long way? Yes and no. Says Joe Gibbons at Towers Perrin: " 'Politics' has become a dirty word, but all it really means is being there when the decisions get made, and understanding the people who are making them." You have to be sincere, though. If you really don't agree with where the new boss is taking the organization, you'd better think about finding a new job--because the odds are good that sooner or later you'll be leaving involuntarily anyway.

To avoid your own personal Friday Afternoon From Hell, you have to look up from your desk, and from the specific skills and habits that got you where you are, and see the big picture. Where do you fit in it? How different is it from five or ten years ago? Why? When you read about new trends in your field, do you understand them? Do new people coming into the company seem to have different, or sharper, skills than you do? What do your subordinates think of you? Jack Snader, CEO of a sales-training firm called Systema in Northbrook, Illinois, notes that the way underlings behave can be a warning sign: "If one comes to me and says, 'What am I supposed to be doing, exactly?' I should know that I'm not communicating clearly enough. That's if I'm willing to put my own ego aside and realize that that's what it means."

Ah. The ego thing. Vital as constant self-evaluation is, it won't work if you can't put aside your ego and look at yourself as others see you. Charles Palus, who coaches executives at the Center for Creative Leadership, says the ability to see where you really stand is "post-heroic. You have to give up being a 'hero.' Be willing to eat some dust. Be humble--which is where true knowledge lies." Niccolo Machiavelli, that career strategist par excellence, said it about 500 years ago in his classic treatise on politics, The Prince. "A prince ... ought to discourage absolutely attempts to advise him unless he asks for it," Machiavelli wrote--and here's the kicker: "... but he ought to be a great asker, and a patient hearer of the truth." Done any asking lately? Done any listening?