Job hunters know their time is limited: You have to be selective about which roles you apply to. But it's not always easy to tell if you're using your time wisely. Sometimes, an employer that posts a public job listing may not be interested in outside talent for their opening.

For example, some government agencies and contractors are required by law to post a position externally, even if they have an internal candidate in mind, experts tell CNBC Make It. Generally, businesses are free to post a "phantom" job publicly, even if only to appear like they're casting a wide net, says employment attorney Tom Spiggle.

So long as a company isn't violating laws against discrimination, that practice "wouldn't be improper, even though it's a little bit slimy," he says.

It's likely not a common practice, according to TopResume career expert Amanda Augustine, but it's also hard to know for sure.

"The unfortunate part is that they're legitimate roles. There's nothing about them that would ever indicate otherwise," Augustine says.

But, she adds, it only takes a bit of networking to get more information about whether a job is really open to external applicants — or at the very least, to make your job search more efficient. Here's how.

Set up a 10-minute informational call

Ask a current employee at the company you're applying to for an informational interview, Augustine advises. Let them know you're hoping to learn more about their role and the organization, she says.

Make it a low-commitment meeting for the other person, so they're more likely to say yes: Ask for 10 minutes of their time over a video or phone call.

It's more effective to talk to someone you know at the company rather than cold contacting a random employee, Augustine says. Even if the person you know is in a completely different department from that of the posted job, they can dig around for information on your behalf or introduce you to someone else at the company who may know more.

If you don't know anyone personally, look for a staffer with some degree of connection to you — perhaps a friend of a friend, someone who used to work with your colleague or an alum of your alma mater, Augustine recommends.

If no one fits that bill, target people in the department you're interested in or who perform a similar function to the open role, she says.

Ask tactful questions

During the call, pose a series of questions that lead to your specific concern about the opportunity's availability. You can inquire pointedly, yet delicately, about the hiring team's progress so far, Augustine says, like whether they're already interviewing people and eyeing any internal or external candidates, or why they're filling the role.

"I don't necessarily recommend going out and saying, 'Hey, do you know if this is only available to internal candidates?'" she says. "But you can definitely go in and say, 'Do you know how far along they are in the interview process?' Or, 'Are they close to having a candidate already? I want to know, do you think it's worth my time to apply for this opportunity?'"

You could also ask whether internal candidates tend to be preferred or whether jobs are opened to existing employees first before being opened up to the public, Augustine says. If roles typically aren't posted externally at first, it may signal that the company has yet to find the right candidate and could consider you.

Ultimately, how bluntly you choose to probe partly depends on your personal comfort level and your relationship with the person you're talking to, Augustine says.

If you're more familiar with the individual, you can be more direct with your questioning, she says. But with someone you don't know as well, she advocates for erring on the side of caution and professionalism with your phrasing. 

A conversation is 'always' worth your time

It's possible that the person you speak to has little if any knowledge of the job you're after. That doesn't mean your conversation is for naught, though — far from it, Augustine says. 

"These informational interviews are always valuable because a 10-minute conversation could save you a lot of application time," she says.

The chat provides an opportunity to get "insider information" about the company's culture, not just one position's hiring process, Augustine says. All of that information can help you decide whether you want to apply in the first place, as well as how to apply, including how to customize your resume or cover letter successfully.

The person you consult may also be willing to provide you a referral for the posting or pass along your materials to the hiring manager, presenting a chance to skip ahead in the recruiting process, she says.

Want to be a successful, confident communicator? Take CNBC's new online course Become an Effective Communicator: Master Public Speaking. We'll teach you how to speak clearly and confidently, calm your nerves, what to say and not say, and body language techniques to make a great first impression. Sign up today and use code EARLYBIRD for an introductory discount of 30% off through July 10, 2024.

Plus, sign up for CNBC Make It's newsletter to get tips and tricks for success at work, with money and in life.