This is an interesting topic and one not just related to M&As but to all types of change activities. Passive resistance can be difficult to spot, but once you are aware data trends may be one way to identify if there are any correlations between recent changes (like M&As) and the observed behavior. If such correlations are identified I would approach the relevant areas, teams and individuals with an open and curious approach, to do exactly what you say here – identify if there is also causality to be found. I would do so by being present, face to face and be curious, not through surveys or top down communication. Spend time with the people and ask questions about their work and there daily activities. I often find it surprising how much people are actually willing to share once you dedicate time to listen to them with an open mind.