“Do you like working here?”
It’s very hard for people to lie about their own feelings. If they take a minute, and finally say something like “….yeah? Yeah. Yeah, I like it here,” they aren’t being truthful to you or, frankly, themselves.
If they answer without thinking, it’s more likely that they are being truthful. “Oh, I love it here.” Read the emotions on their face, listen to how they say it. If they use the word love instead of like, follow up and ask them why. If they are passionate in their reasons and light up as they go on, their account of what it’s like to work there is usually an honest one.
If you interview with multiple people at that organization and ask them that same question, you will quickly discover if it’s just that one worker who is happy or unhappy or if it’s the whole company that feels the same way.
Your interviewer’s answer to the question “do you like working here?” is probably one of the most honest things you will hear in the whole interview.