How To Program Selective Plugin?

I have developed a plugin to manipulate categories that display in a subnavbar in the header of my theme. I am using the 'wp_list_categories' hook to limit the categories that show up on the navbar.

But this is the only section of the page where I want this filtering to take place. I want the default list all categories behavior to continue in the right sidebar, for example.

How can I somehow make the plugin aware of whether the list categories call is being made in the header versus elsewhere in the construction of the page? I can't find anyway to make a plugin aware of where in the page construction process it is being called.