I'm looking for a tool that could help me understand the layout of a closed source Mac OS applications in term of Cocoa controls. I saw this for in Firefox with the 'inspect' feature (for HTML/CSS); I don't expect it to work for every application because of the variety of programming languages and custom classes.
You can use a usability tool from Apple to see the structure of most user interfaces; try the Apple developer tool Accessibility Inspector. I believe it will do what you need.
The latest Xcode download may not include this tool as standard. You may need to download an additional package to find this tool.
I've not come across anything of this sort, howwever the only solution I could think of would be to view the app's package contents and find the NIB files, of which there can be just a few or hundreds, dependant on the app in question.
Using a tool such as NIB Decompiler (although I've never personally used it) to then view the decompiled NIB within InterfaceBuilder/Xcode. The project has not been updated in a few years and may not be up to date with the latest version of Xcode.