I modified this answer of mine slightly to meet what you are looking for.
How can I improve my judgement, and better discern how managers will behave before I join a company?
You probably shouldn't directly ask this question during an interview. You can, however:
- Ask what favorite/least favorite things about their job are. People who have to really think about what their favorite parts are but jump immediately to least favorite items can be telling. You can also ask deeper questions about each answer for follow-up which should be natural, give you your answer, and get honest feedback. Make sure to ask both together - this means you are much more likely get honest negative feedback. If you just ask "what do you dislike about your job" it comes across much differently. "Oh, so it sounds like John Doe is a good manager?"
- Probe deeper with non-managers. People you will be working with will be more likely to give you the honest truth. Even moreso in casual environments, so if you are walking around on a tour or transitioning between interviews, use this time to ask these questions as people tend to be less aware of "gotta sell my company!" in informal environments during an interview. Focus on the casual times.
- Ask about things which the answers naturally give you insight on this. Questions like, "what sort of things does manager X do to facilitate employee engagement?" are perfect. People will answer in such a way which helps you realize if people enjoy working. There are a lot of questions you can ask like this, and following up on them will get you your answer.
- "We don't really do anything, but honestly it's a great company to work for and I enjoy it". "Yeah? So you like your manager then?"
- "We do all sorts of those I guess." "hmm... so you must like working for John Doe then?"
- Write out questions in advance. This seems like a no-brainer but most don't seem to do this. If you want to comprehensively determine information about the company you are interviewing with having a list of questions is really beneficial.
- Ask about collaboration/teamwork. Simple questions like, "what are some challenges your teams face?" can go a long way to opening people up. People like to complain about coworkers and phrasing a question like this can have some really revealing answers. As with before, just make sure to followup on whatever answer you get. Even asking a question like, "do your teams ever do events outside work for fun?" will give you great insight, even if you ultimately don't care about the answer either way, because it's nearly impossible to answer a question like that without having some conversation about how well everyone enjoys working together.
- Pay attention to overall mood/atmosphere. You can use this to either frame interesting questions or just be aware of it.
- Ask, "you've been spending time trying to convince me to work here. But if you had to convince me not to work here, what would you say?". If you feel comfortable with your interviewers, this can definitely be an interesting question.
- Glassdoor/LinkedIn. Both are easy to get some insight, be careful putting too much trust in them though. People are much more likely to say, "X sucks!" than "X is a wonderfully normal place to work, everyone generally enjoys their jobs and have no significant complaints." Look for trends here, not so much individual data points.
Basically, with some pre-determined questions intending to ask about the issue indirectly and a few follow-up questions, you can get someone in a place where they are talking quite candidly and openly about how well they enjoy working for the manager.
The keys are finding non-managers, asking questions with good followup, and getting them in "non-interview" environments, whether lunch during the interview, coffee outside it, Linked-In, or some other environment.