'''Note''': I assume the xmodmap executable is in your {{PATH}}, so that you just have to type xmodmap. If it is not: xmodmap can be found at:

/usr/bin/xmodmap

To get a first impression what keycodes are mapped to which keys, just type

xmodmap -pke

which gives you a list of all mappings. For example, the line

keycode  57 = n N

means that the keycode 57 is mapped to a lowercase "n" if no extra key is pressed, and an uppercase "N" if shift is held.