A big red flag is anyone who's never fired someone or never had a bad fit. The key to me is the follow-ups you ask:
• If you've never fired someone, what was the closest you've come? What was the outcome of that situation? Did they quit or did they improve? Why'd you wait for them to quit instead of addressing the issue directly? What made them improve?
• If it was a performance issue, how did you know they were underperforming? What steps did you take to help them improve?
• If it was a culture problem, why did it get missed in the hiring process? If they were initially a good culture fit, what changed?
• How do you identify low performers? Objectively? Subjectively? A mix of the 2.
The caveats are if a) you really really want this job or b) you really really need this job. In those cases, you should only be asking questions that you think make you look good to the hiring team.