How to investigate "firm culture"

I received an email from a reader asking the following question:

I just read your post on firm culture and I was wondering what you did
to discover what the culture was like at your firm. I used to do IB so I'm
highly cynical about what any firm/company tells prospective employees about
life there. I'm curious as to what type of investigation you did.

I wrote the reader a scurried and typo-riddled response by email, but realized that others may be interested in my opinion also. Here's an edited version of what I wrote. I also tried to categorize the different sources I used to scope my firm out, and here's the result:

1. Personal networking. The small firm ("Small Firm") I worked at before coming to my current one ("Big Firm") had 3 partners, and one of those 3 partners was a former associate at Big Firm. I worked hard to maintain a good relationship with this particular partner, and he was sad to see me go but very excited about my wanting to work at Big Firm, which he had left on good terms. He gave me first-hand knowledge of the kind of culture Big Firm had. In fact, he took a lot of what he learned at Big Firm (in terms of management style) and applied it to the atmosphere we worked in at Small Firm, so I already had an idea of what to expect.

2. I was not shy about asking questions during the interview process. I asked, up front, "What are the firm's billable hour requirements?" When I got vague answers, I told one senior partner, "I'm not getting a clear answer from others on the firm's billable hour requirements. Can you give me an idea of what to expect?" Answer: "You're not getting a clear idea because we don't have a requirement. Most people come in at around 1900 to 2000, some are a lot more and they're going to get bigger bonuses, some are less and they'll get smaller bonuses." And on a related note, I got a general vibe from these senior partners that it was okay for me to be honest and up-front with them, and ask them direct questions. If I had gotten a different impression, that direct questions were not really welcomed, that would've given me a different perspective on what kind of firm this is.

3. Talking to younger associates at the firm. Although I asked many direct questions to the senior partners, there are some things you absolutely should use your judgment about and not bring up with them. These touchier subjects (e.g., do the partners really care if you leave the office earlier than they do?), I saved for my conversations with the younger associates I was introduced to. I realized that they too might try and whitewash things for me, but at least I was aware of that as I listened to them describe the firm. Ultimately, I simply got the right vibe from them while talking, and simply used my instincts to guess that they were not masking an ugly reality. As it turns out, my instincts were right. Also: it's significant that I was even given "alone time" with these junior associates, and openly encouraged to talk to them about the firm. If a firm won't extend the same invitation to you, consider that a red flag.

4. My earlier dealings with attorneys from this firm before interviewing here. I had worked with attorneys at this firm before, in cases in which our firm was opposing counsel to them. And I always found this firm's attorneys to be professional, respectful, genuinely nice people who I liked even though we were on opposite sides of a case. There are plenty of firms in my city that fail to give me this impression. (And frankly, my dealings with these attorneys on these past cases is in part what led to my eventually being hired here).

5. Other miscellaneous observations I made during the interview process. While interviewing, I noticed that people wore business casual around the office. As I walked through the halls, people were smiling, joking with each other, seemed genuinely happy to be doing their jobs. I was told that the firm is "dead" during the weekends (i.e., no compulsory weekend work), accepted that as an honest representation, and it turned out to be true. (Note: one way of verifying the accuracy of such a claim is to literally drive by the office on a weekend, IF you know that the firm's associates have a parking area you have access to. I had no such access for this firm, but you might.)