How to get a list of modules implementing hooks? Is it possible to get a list of defined hooks for debugging purposes? 

I have implemented mymodule_datalayer_meta() for datalayer's hook_datalayer_meta(). However, as far as I can tell, it's not firing. I'd like to look up a list of hooks that modules implement so I can see if my module is actually implementing it or not.
You are looking for the module_implements function


  Determines which modules are implementing a hook.


Use this function in the devel/php form, for example.

dpm(module_implements('THE_HOOK'));


This way you can see easily if your module implements 'THE_HOOK'.