How to understand a company’s culture before accepting the job?

Would be difficult unless you know people in the company.

As someone in HR, I would suggest that you post a scenario based question. For example: questions relating to how the management manages their internal and external stakeholders, what is the turnover rate for the role or why was nobody internal consider for the role (if it's not a fresh role), etc.

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Once you've asked any questions along this line, read subtle cues (these are straight giveaways!). Next, listen to the way they approach the question - was it from a first principles thinking or was it dismissive or complete avoidance?

Impossible unless you know someone from that company

Sus out the boss character and immediate team members