So, the esteemed exor674 made a dev tool script called lookup-routing that is in $LJHOME/bin/dev which can be used to find out what handler is in charge of a given URL path. I've taken the documentation for it and put it into the Routing and Template Cookbook with some extra explaining:

Routing and Template Cookbook: Find the handler for a URL

(Maybe we could add a routing tag to this entry?)