Getting Menu_Id for a Menu-Block placed in a Region of a Theme

Hello!

This may be OT here - in case it is, maybe you can point me to the right place to post this.

I'm working on a theme for a non-profit website. I've been googling for a whole day without finding a solution.

I want to achieve the following:

Menu-Blocks that are placed in a certain region of my theme need to be processed to change their markup. Processing is not hard to do. But how can I get the following info from drupal:

1. Is there a Block containing a Menu in a certain region?
2. If so, what is the menu_id of this menu, so that my theme can process the menu tree?

Any help would be appreciated a lot.

Thanks

Jörg