A CEO says he always does the same thing before offering anyone a job

There's one more thing you need to do after you ace the interview, says Duolingo CEO Luis von Ahn (pictured).Luis von Ahn

When it comes to hiring people, Luis Von Ahn, CEO of the language app Duolingo, has one strict rule.

"We have a big 'we don't hire assholes' rule," he tells Business Insider.

So, how can you ensure you don't hire someone who's a bad fit — or a massive jerk?

"That sometimes comes out in interviews, but that's where people are usually on their best behavior," Von Ahn says.

Instead of banking on interviews, the executive says his company relies heavily on references, which most candidates readily provide.

"You'd be surprised how much comes out during that process," he says.

"At first I thought when we did reference checks, it would be like a rubber stamp," Von Ahn says. But he soon found that's far from the case.

Von Ahn, who is actively involved in Duolingo's hiring process, estimates that about 30% of the people the company has done reference checks on end up not getting a job offer.

The reason isn't because references are openly bashing job candidates. "Nobody says, 'Oh, don't hire this person, they're an asshole,'" Von Ahn says. 

Instead, Von Ahn reads between the lines.

"You ask questions like, 'How good are they at working with others?' They won't say they're bad. They'll say, 'Well, they're pretty good, but it depends on the team,'" he says. "When they say stuff like that, it's like, oh, I get it. Okay. For some teams they work well, for some teams they don't. That's not good."

This is a good lesson for job seekers. The next time you're asked to provide references, make sure you list people who can truly speak to your strengths.