It happens a lot that you want to hire people but do not have the knowledge and experience to tell if they are good at what they do. You need a logo, how do you tell an exceptional logo designer from a competent one?
For growth hacking, proven experience is best, but everyone has to start somewhere. Look for a background in and understanding of consumer psychology, user experience design, social media marketing, A/B testing, and analytics. Have they read and understood books like Influence, The Power of Habit, Nudge, Ideas That Stick, Predictably Irrational, The Laws of Simplicity, and The Paradox of Choice? Do they have a firm grasp of UI/UX concepts?
If you want to give someone a trial run, see what they can do to move a KPI in the right direction. Keep in mind, though, that even at its best, growth hacking involves a lot of trial and error. AirBnB touts their successes, but they (like everyone else) have a lot more failures (that is, changes that do not improve their KPIs) than successes.
Even more important is demonstrating the ability to formulate a test, evaluate the results, and generate knowledge from it. In my book, a change that lowers a KPI is only truly a failure if you don't learn something from the experiment. If you lower the price and fewer people buy, it tells you that at some level your customers want to be buying a premium product. If you make that change and learn that lesson, the experiment was a success. If you make a change and have no idea why it generated the results it did, then that is a real failure.
Of course, I also agree with Alan Peters: do not even bother to think about growth hacking until you are sure you have achieved product/market fit and are ready to scale.