How do you know whether a keybinding already exists?

[Elena <egarrulo@gmail.com>]

Hello,

I'd like to trace whenever a key-binding is being overwritten, and
eventually forbidding some overwrites.

My understanding is that both "global-set-key" and "local-set-key"
rely on "define-key", therefore I've written this advice:

(defadvice define-key (around protect-bindings
                              (keymap key def)
                              activate) ;; I'm using an "around"
advice to eventually disable the overwrite.
	(when (key-binding-exists key keymap) ;; For "key-binding-exists",
read on.
		(message "Overwriting %S binding." key))
	ad-do-it)

I don't know how to write the "key-binding-exists" function.

Or it would be better to follow a different route altogether?

Thanks.