Is there an application to know which key is bound by which process and therefore not usable in another application?
Find out which application listens to specific key bindings?
x11;keyboard shortcuts
There is no competition between applications of keybindings. Either your X server catches something (in most cases your Desktop) and reactions on a key (like pressing the volume up key) or the key is passed to the current application (i.e. the one that has focus). This is different from what I remember from Windows where you could have hot-keys that went to applications that did not have focus.So you don't need to know which Alt+somekeyis already used by some other application, you only need to know if your desktop already catches it and does something with it. If an application needs to know about a keyboard event e.g. pressing the Forward key on a Mediakeyboard, then the Desktop should catch that event and notify the application.