How to evaluate the company culture before joining?

When I'm getting interviewed I look for a few indicators of culture. I ask what type of backgrounds people have (are they all university, or some self-taught etc), if they promote people internally, have formal mentorship programs, how they hired the last person (because they are xyz race or random stranger), if they lunch together, what their junior to senior dev ration is, how their code review process works (do they listen to other opinions) and how frequent overtime is.

Interestingly overtime question are pretty easy to read, if they don't look you in the eye when they answer, it is a red flag. If you're lucky, the interviewer will say something like "I have kids I have to pickup, I never do overtime" - then its clear you've got a good worklife balance.

If it's a small enough company, I would definitely suggest interviewing the founders from the perspective of an investor. Your line of questioning should be focused on asking them/yourself, "why should I invest my time in your company?"

This should raise all sorts of questions like, what's the value-prop of your product/service, what's your go-to-market plan, what is your experience building and scaling companies, etc.

If you can't personally be interested in the company after speaking to the founders - I think it's likely that it won't be of much interest to you.

Glassdoor is often useful. Sort by "low" and read with a grain of salt. If it's large enough, specific issues will bubble up.

How are you feeling marginalized?