NEW YORK (FORTUNE) - Dear Annie: I was among the top five salespeople at my software company last year, and I'm pretty sure I'm drastically underpaid. I really like working here, but I suspect I'm not being compensated fairly because I'm the youngest person in this role. How can I verify what other people in my position are making across the industry, to support my argument that I deserve more money? --Super Closer

Dear Closer: It shouldn't be hard to get a fairly accurate sense of your market value. First, websites like Salary.com can give you guidelines on what people are making in various jobs and geographic locations.

To back up those numbers, check out help-wanted ads -- in trade publications, on the big job boards, and at specialized boards for salespeople such as Salesanimals.com -- to see what pay ranges they mention for people with track records similar to yours.

If you're acquainted with any recruiters, call them. These folks are in a position to give you the straight, up-to-the-minute skinny on who's earning what.

If it turns out that you are indeed at the low end of the pay range, you can certainly mention that when you ask for a raise. (Your boss probably already knows it, but may not be aware that you do.)

But don't make that the main part of your pitch. Instead, spend most of the conversation pointing out how good your numbers are, what great new clients you've brought on board, and so on. Gather all the facts about your terrific performance and write them down so you don't overlook any. Emphasize that you are asking for a raise, not because other people make more than you do (someone probably always will), but because you're worth more to the company than you're currently making.

Don't be too surprised, though, if your research reveals that you're already earning just about what you should be. Consider some fascinating findings from a new poll, wherein Salary.com collected pay data from about 14,000 employees and 400 human-resources managers nationwide. Among the 1,600 people surveyed who said they think they're underpaid, the researchers' analysis says that over 80% are not only already earning their real market value, but -- surprise! -- some are actually paid more than their job description warrants.

"People's pay is a sensitive topic, and most people probably don't want to be told that they're already being paid what they deserve," observes Bill Coleman, a Salary.com senior vice president.

"There are a number of reasons why employees may mistakenly think they're underpaid," Coleman says. "For instance, we found that many are earning so much less than their apparent market value that many of them had probably been 'overtitled' " -- that is, given a fancier title than their actual job description merits.

"Over-titling" was a common practice in the tough financial climate of the past few years when, Coleman says, "many people were offered trumped-up job titles in lieu of salary increases. As a result, their actual experience level and value to the company may not be on a par with the salary they expect based on their title."

Indeed, when Salary.com compared employees' job descriptions (not necessarily their titles) with what they currently earn, 30% were found to be "overtitled," while nearly 35% were earning about what they'd be worth on the open market, and 20% were overpaid relative to what they could make elsewhere. Only 15% were actually underpaid.

Obviously, this can make it hard to figure out how much of a raise you could get by changing jobs. If, for example, you recently got bumped up to assistant vice president but without any increase in your real-life responsibilities, you'll get a distorted view of your market value unless you focus your search on jobs whose descriptions match what you actually do, rather than on those whose titles include the words "assistant vice president".

Of course, maybe you really are underpaid. But if instead you're like that 20% in the Salary.com study who are actually earning more than their job warrants, I won't tell if you don't.

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