There are no uniformly valid predictive indicators for successful hires. However, there are certainly valid interview practices for assessing communications skills and potential project fit.
First, communication skills are often self-evident during an interview—assuming the interviewer has sufficient communications skills to evaluate them in others. Some ways to validate candidate communications skills include:
- Assess how well people speak in interviews.
- Assess how well people write cover letters, source code comments, prior publications, or interview exercises.
- Assess the candidate's active listening skills.
- Assess how effectively a candidate transfers ideas in layman's terms, especially regarding job-related or technical concepts.
None of these things will guarantee that the person will communicate effectively if hired, and people with disabilities may perform poorly in some of these areas despite otherwise good communications skills. Still, if effective communication is a basic job requirement, the only way to test for communications skills is to actually attempt to communicate.
From a project management perspective, your main concern in the interview process ought to be whether the candidate is a potential fit for your project management framework and project culture. It is generally someone else's responsibility to vet the person for the technical skills necessary to complete deliverables; even if you happen to be a technical subject matter expert, your role is managing the project, not vetting someone's source code, so focus on how that person is likely to fit within your core areas of responsibility: project framework practices and project controls.
Investment companies will be the first to tell you that past performance is not a guarantee of future results, and most interviews are really just hearsay anyway. Nevertheless, there is a lot of value in inquiring after a candidate's experience with your particular project management framework (especially when using agile frameworks) and then listening to how the candidate answers.
If you're a Scrum Master, there may be value in hiring a candidate who already has experience with a relevant Scrum role. A candidate who understands (and can articulate!) the purpose of a standup meeting or how to play Planning Poker is probably a more likely to be an effective self-organizing team member than one who is used to needing written authorization to blow his nose.
If you're a waterfall practitioner, there's probably more value in hiring someone who understands the planning and reporting requirements you need than someone who prefers to "get 'er done" without all that tedious project management stuff.
That doesn't mean you can't hire someone who isn't a project management expert, or who doesn't have experience with your particular framework. However, all other things being equal, an understanding of the team's project management framework and practices (and, perhaps more importantly, a shared appreciation of those practices) is going to be more valuable to you as the project manager than anything else you could ask.
There aren't any guarantees in hiring. Even if someone is a perfect fit, they may get a better offer, learn to detest their teammates, suffer a health crisis, or decide to enter a monastery as a career change. Life is unpredictable.
All you can really do is identify what's important to you in your specific organizational role, and then do your best to select for the characteristics that make your job at least possible. If you get it wrong (and it's even odds that you will) then do your best to honestly assess whether you selected for the wrong things or whether it was "just one of those things." If the former, fix your selection process; if the latter, increase your statistical sample until you achieve the results you want.