I guess that depends on your powers of observation. But even if you trust in your powers of observation, it is an excellent opportunity to compare what you observe with what they say. If things don't mesh, you have to wonder.
Agreed. And remember you're only seeing a little snapshot in time, so there are probably quite a few questions you could ask regardless of your observational powers.
Do you eat lunch together? Is dress always this casual? How often do you refresh your equipment? How often do people work in groups vs solo? Do you encourage working in pairs? How geographically distributed are project teams, typically? Do people take personal calls at their desks? How do you manage shared resources like conference rooms? At my last job, everyone stood up and belted show tunes for a 1/2 hour at tea--do you have such a program?
What you see visually doesn't tell you anything about the team's thoughts on testing, automation, how/if they do code reviews, what their thoughts are on working remotely, if they care about what hours of the day people work, etc.