How to find out which keys are free for my own shortcuts?

In Blender 2.55 I want to have a shortcut to Pose > Motion Paths > Calculate, but I don’t know which key is free.

Can somebody tell me which is the supposed way to making your own shortcuts without breaking another?

I don’t know if there is a easy way. I would do this way: Change the view to Outliner and you see a combobox that you can change to “Key Maps”. Now open “Pose” in the tree and you see all the keys already used. (Currently I have changed some of them to other keys and they continue to show here the old values, keep this in mind)