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.
I’m currently working in an integration were differences of culture between 2 countries haven’t been looked at after one year since the acquisition. I just arrived on the new site and the first thing I was surprised about was how still, after one year, both teams don’t really understand cultural differences and the different perceptions each team can have of the other.
I just asked an integration coach about any tool about culture difference, so here is the website if you want a look at different cultures between countries. https://www.hofstede-insights.com/fi/product/compare-countries/
This is a starting point. Like others have said, checking with employee really brings the knowledge of the different culture and sometimes, talking to other people who have done the same culture integration can give you some insight, but at the end of the day, each company have their own culture and it’s with building relationship and communication you can really get each other.