How can you tell in an interview if a company's culture is authentic?

Almost all companies present an open, caring, nurturing culture, with good honest management who care about the worker, very few have it. This place covers for, covers up, defends & lies to protect racists, bigots, sexism, misogyny & hate speech, while attacking, lying to and about, threatening those who point it out.

observe employees at work, and smile at them. see what they do back.

You really can't.

By getting answers to the questions I ask and not have them beat around the bush.

The problem is every company wants to be honest but they can't. Requiters lie and human resource will tell you what there values are but the supervisors that you work for just want to cut corners to make a dollar for the company other than that I have never in all my time of working in the last 30 years ever seen an honest company they dont EXIST

The body language and demeanor of the underlings around the leader.

I ask who has recently been promoted, and why. The answers point to what the company values, which in turn speaks to their actual culture. I can then compare that to what they describe as their culture on their website, and determine if they're being authentic.

Not in my personal experience. Psychologically, it's natural to exhibit positive influencial behaviors when attempting to gain someone's trust or acceptance. Sometimes it's intentional, sometimes not. From body language, tone of voice, micro-expressions, language choice and more, your interviewers are working from the outset to give you the impression of their company culture as they want you to percieve, rather than as it actually is.

If they really push their culture,it's being forced.

By the answer to your questions.

Difficult during an interview, but if there’s employees around you can talk to before or after, you can get a better sense of the company’s culture.

It’s only authentic if the leaders are authentic. Lack of authenticity is easy to spot.

Ask non-standard questions like: what social sports do people play in the office? Or what movies get quoted around the team?

Do their managers look half dead while the corporate people look healthy