Outside of a couple of phrases I consider clear-cut red flags (such as "we're a family here", "we work hard and play hard", etc.), what I'm really looking for is a bit squishier and not tied to particular phrases and such.
Things that I think indicate a poor fit for me include places where it's expected that people work more than 40 hours per week (I have no problem doing that in unusual circumstances, but if it's part of routine practice, that's a problem); where it's expected that you'll take part in extra-curricular activities; where they take agile methodologies too seriously; where there's too much social stratification in the workplace; and such.
Concrete red flags include using leetcode tests, using personality tests, absurd interviewing hoops (too many interview rounds, too many days to complete the interviewing process, etc.).
Not being interviewed by (or at least meeting) the members of the team I'd be working with is a bad sign, as is the interviewer not knowing the position I'm being considered for. The interviewer not answering my questions (or being vague or evasive in their answer).
I pay attention to the other devs I see. If most of them look unhappy or stressed, that's an enormous red flag. If in-person, most of the desks in use being pretty bare (little in the way of knick-knacks, personal photos, funny or interesting things hung on the cube wall, etc.) is bad. A lack of enthusiasm about what the devs are working on is bad.
Immediate showstoppers are having an open office plan or hot-desking.
Having too fancy of an office is a bad sign, but more of a yellow flag than red.
I'm sure that I've omitted a bunch of things here. I don't have a literal list in my mind. What I tend to do complete the interview process and then put it out of my mind until the next day. Then I go through my notes and examine to totality of what I observed, both things that pleased me and things that didn't.