Ask each member of your team to write down a list of what frustrates them about the code. The symptoms of code complexity are things like emotional reactions to code, confusions about code, feeling like a piece will break if you touch it, difficulties optimizing, etc. So you want the answers to questions like, "Is there a part of the system that makes you nervous when you modify it?" or "Is there some part of the codebase that frustrates you to work with?"
Each individual software engineer should write their own list. I wouldn't recommend implementing some system for collecting the lists – just have people write down the issues for themselves in whatever way is easiest for them. Give them a few days to write this list; they might think of other things over time.
The list doesn't just have to be about your own codebase, but can be about any code that the developer has to work with or use. You're looking for symptoms at this point, not causes. Developers can be as general...