This is much more difficult. You basically have to search the keymap files of ST2.
I personally, on plugins that I release, try to bind all of my commands to a sequence. I use "ctrl+shift+b". So everything is ["ctrl+shift+b", "ctrl+shift+something else"]. That way I am only taking one shortcut away and just building combinations off of that.
ST2 uses so many shortcuts that it is becoming increasingly more difficult to find shortcuts. Taking into account all of the plugins that you may install that eat up more shortcuts, it is hard to say which are free.
That is why I also have started to shy away from releasing plugins with shortcuts defined. Instead I create commands in the command palette and let the user create a shortcut if they really want one.
If you are writing a command or plugin for personal use, then you can overwrite shortcuts that you don't use, but if you plan on releasing it, you have to be very more careful.