There is not a central location where this information is stored. Essentially there can be many programs that are always running in memory that are looking for keys they are designed to watch. For example, Windows 7 has certain
keystroke commands. But I also run a piece of software called DBLCLICK which offers the ability to associate keystroke with winamp commands, I don't use that portion but if I did that particular software would be watching for the keystroke combinations I define there and would perform specific actions in winamp when it detected them. And when I run some video games they define certain keystrokes if the game is the active window.
You would have to know every program running in memory and what keys are used by each. It's probably easier to just try the key before you chose to assign it.