I do a few things:
Before joining any job I get some background on the executive team. That will tell you a lot about the culture, and let you know if the current team still has the founder or not (which can make a big impact on company culture).
That will tell you a fair bit, if you brush up on their background you can get some approximate sense of how it may influence company culture. I also look at where they are in life, as this can often indicate where they may lean into their philosophies in decision making. You can find patterns here sometimes.
Then, I do similar for the immediate time I'm working for when I'm hired. That will give you a sense of their personalities, at least.
Next, I really dig into how the company presents itself outwardly, and the general tone they carry. This one is harder for me to describe, but I have noticed that more "stuffy" companies tend to have a certain vibe to their presentation, for instance.
Finally, try to estimate where in the company lifecycle the company is. Is it new and unproven? Is it new yet has product market fit? Is it an older company climbing its peak or has it hit peak and is now largely to the point of being as big as its going to get or may even be plateauing?
these questions usually get me a good enough sense of company culture I haven't felt I've ever been surprised by company culture once I arrive at a given job.
Extra credit you can simply have an off the record conversation with your potential co-workers on LinkedIn or something, but not everyone answers cold questions about what its like working at X