Hi Jan, I think the best informal cultural DD is talking to people through the organization. As well as this reviewing recent 360 feedback for key managers and executives can give insight into the culture.
Some HR consulting firms have their own framework and tools to assess cultures of buyer and target. The tools can help to visualize the major gaps and red flags. Change impact assessment needs start early -identifying key changes, assigning change agents and measure the sentiment of people.
Other the above points raised, I believe one can also do some checks with ex-employees with the organisation to see what they have to say (it is much easier to connect with people today through platform like linkedin). Believe one can hear what he/she would have heard from existing employees.