Consider this example "session stack": vim inside bash inside tmux inside a terminal emulator inside my desktop environment.
Any given key or chord may be mapped at any one of those levels. Sometimes I want to know "what does ctrl-XXX do" -- perhaps because I want to map it to something myself. If I'm lucky, pressing the key does something obvious. Not so lucky, and I must resort to brute force: look up the documentation for each piece of software in the stack, figure out how to display its active keybindings, then test the key in question against those bindings.
Is there a better way? Can I send a key and "watch" which process handles it, and how?