I have a few questions I ask which often tease out the red flags:
* What hours did you and your team work in the last month? (anything that indicates long hours are frequent is a red flag, not because long hours are intrinsically bad (though they are), but because it's usually being used as a proxy for measuring productivity).
* What is your policy on remote working? Who do you have remote working currently and under what circumstances? ("no remote working" is ok for me if there's a non-bullshit reason but YMMV).
* Who is currently your best or one of your better developers? How did you come to that conclusion?
* What characteristics do you look for in clean code vs. technical debt-laden code? How do you handle and plan for paying down technical debt?
* Who reads your team's pull requests? (worst answer is "nobody", 2nd worst answer is "the team").
* Have you read any of my code? What did you think?
I'm usually not looking for a perfect answer to all of these questions, just avoiding the companies that give an impression of widespread systemic dysfunction in their answers or a manager who is clueless.