Then finally, happy people generally don't go out an write a huge essay on why things are fine. unhappy people will go on-and-on about every bad thing to anyone who listens.
Thus, we're left with disorganized reports on various sites to glean culture. Eg HN comments on the story about Amazon suing Brian Hall. https://news.ycombinator.com/item?id=23461326
For those looking to move within the job market, it would be worth it to talk to a tech recruiter for a few hours who's job it is, is to know company cultures. (Traditionally, if you, the new employee that they managed to get into a company with a bad fit, leaves before 90-days are up, they don't get paid.)
IMO this is very good advice. Having been on both sides of the equation, I think a good tech recruiter (emphasis on good) brings a huge value to the process. As a hiring manager, I had one guy I knew I could rely on to bring me really qualified candidates every time. His own filter was excellent, and he only brought me people that were a good fit. I almost didn't need to interview them. Other recruiters were terrible, one even sent me a candidate who had faked their first phone screen so they weren't even remotely qualified for the job.
That is the guy you want to know. He has earned his reputation. And it cuts both ways, too -- his success isn't just when the company is happy with him, but when candidates are too -- all the best ones are his clients, after all.
Dig up some fake creds on BugMeNot and poke around Glassdoor