Over the last decade, I’ve started three digital agencies, become the CEO of Profit Labs, and interviewed more than 500 candidates for jobs across those companies.

I’ve learned that predicting how someone will perform once they’re hired is about asking the right questions.

And one question — more than any resume or reference check — has consistently told me whether a hire would become one of my best or one of my worst.

Across hundreds of interviews, it’s been my most reliable predictor of long-term success, saving me more money than any hiring tool.

I used to ask candidates about their “zones of genius” — including excellence, competence, and incompetence — to glean what they’re great at, what they can do decently, and what they struggle with. But the word “incompetence” often put people on the defensive. I kept getting rehearsed answers when I wanted honest ones.