In October, Tom Greene was invited by email to interview for a vice president position he had applied for through CareerBuilder.com. Before accepting, the sales and marketing executive called the search firm that posted the ad to ensure it was indeed a job opportunity.

Mr. Greene didn't want a repeat of two years ago, when he agreed to an interview in the same circumstances only to find there was no position available. Instead, he had received a pitch from a career-marketing service costing up to $10,000, starting with a $6,000 upfront fee.

Tim Foley

This time, the 53-year-old was assured by phone that the job was real and he wouldn't be asked to dig into his wallet. But after driving a half-hour from his home in Colonia, N.J., to meet the firm's recruiters, he says he found himself once again listening to a pitch for a career service, without any prospect of a job.

"It's extremely frustrating," says Mr. Greene, who weeks later was laid off from the financial-services company where he worked. "You get very skeptical about the job market."

If you're launching an online job hunt for the first time in a while; take caution. What may look like an ad for employment may lead to something entirely different, like a hard sell for career services or job-training manuals. Or worse, it might be a plan by identity thieves to get you to share sensitive personal information via "phishing" expeditions. Some of the job postings -- sometimes for positions long filled -- also could be from recruiting agencies looking to collect résumés.

The problem of job postings that aren't what they seem is adding to the frustrations of the more than two million recently laid-off workers who are competing for an increasingly limited number of jobs. The good news is that there are several tip-offs that indicate an ad is likely to lead you down the wrong path. And as long as you don't give out any private data, getting duped into responding to a fruitless job ad will likely only cost you time and energy.

Last fall, for instance, Mary LaFleur Langdon searched for children's photography and teaching-assistant jobs near her home in Milford, N.H., at craigslist.com. She answered several ads that didn't name the hiring companies and received email responses all asking her to complete the same application at Career-Hub.com, which no longer exists.

After filling it out, Ms. LaFleur Langdon says she received an email instructing her to return to Career-Hub.com and provide additional information. Once she did that, she says, she was directed to a Web page advertising various jobs and no explanation as to the fate of her application. She completed the form again, and this time she answered "yes" to a question asking if she was interested in furthering her education. The result was an ad for an online university. "I was pretty angry," says Ms. LaFleur Langdon, 46 years old.

Some recruiting agencies post ads for eye-catching positions to attract applicants for the less-desirable jobs in their portfolios, says Steven Greenberg, publisher of jobs4point0.com, a job site for experienced professionals. Often the jobs were already filled, or they're a composite of several ads. Recruiting firms do this because "they're looking to gather as many résumés as they can for posting the fewest ads," says Mr. Greenberg. Later, they'll pitch these low-interest jobs to people whose résumés they've collected in hopes they are desperate enough to take a less-attractive position.

Meanwhile, independent recruiters often dilute the marketplace by posting their own ads for the same position, says Elaine Rigoli, editor of the Fordyce Letter, a trade publication for recruiting professionals. They're competing against one another for a fee that comes with placing the winning candidate, she explains. The ads contain different language and contact information, giving job hunters the false impression that there are several openings on the market, she says. Still, it can be beneficial for job hunters to apply to them all since recruiters don't always agree on who makes an ideal fit for a position, says Ms. Rigoli.

The ads to most watch out for are those pretending to offer a job but are really trying to get you to give up personal information, such as a bank-account or Social Security number. They often mimic real postings, with some featuring company names and logos nearly identical to those of actual employers. The individuals behind these postings may even exchange multiple emails with job hunters to build up trust.

To be sure, many job Web sites review submissions to try and prevent inappropriate ads from getting published. A spokeswoman for CareerBuilder says the site employs a team to screen job postings and verify their information. But job boards can't spot everything ahead of time, site publishers say. "Fraudulent activity across the Internet continues to evolve and mutate," writes CareerBuilder spokeswoman Jennifer Sullivan Grasz in an email.

Of 132 job-site publishers polled in October, 39% said they frequently find ads that peddle sham investment opportunities or request personal information under false pretenses. More than half said the review process regularly turns up ads to get job hunters to buy services or products, according to the survey by the International Association of Employment Web Sites, whose members power more than 60,000 job boards.

Some deceptive ads end up on job boards anyway because "scammers learn each time how to break through whatever the filter is that's knocking them out," says Gerry Crispin, co-founder of CareerXroads, a Kendall Park, N.J., consulting firm that specializes in recruiting technology. "They come back over and over again trying new techniques." Big job boards are particularly vulnerable, he adds.

So how can you tell if a job posting is insincere? One sign is that it lacks details about the hiring company and position, says Pam Dixon, executive director of the World Privacy Forum, a nonprofit group in Cardiff by the Sea, Calif., that specializes in privacy matters. Such an ad might describe an employer as a "major technology firm" rather than cite annual sales or say what kind of technology it produces. It also might offer a vague job description or list a salary range spanning more than $50,000. Genuine ads typically target applicants who have a specific amount of experience and pay salaries commensurate with their backgrounds, says Ms. Rigoli of the Fordyce Letter.

If you're unsure whether an ad is sincere, you can protect your identity when responding by providing a resume with a post-office box address instead of your home address, says Ms. Dixon. You might also list just your initials in the document and not your full name. Further, consider using a disposable email address to prevent spam from clogging up the one you normally use. If a business address or company name is provided, and it's a name you don't recognize, search for the employer's Web site to learn more about it. You also can check for any complaints filed against it with the Better Business Bureau at bbb.org and consult with people in your network.

Write to Sarah E. Needleman at sarah.needleman@wsj.com