To find keycode run
Code:
xev
position your cursor on the window that pops up (ignore output on terminal about mouse movements). Then press the key and note the output in the terminal.

To set the keycode to another value

Code:
xmodmap -e "keycode xx = some-key-value-here"
where xx is the keycode of the key and an example of some-key-value is Alt_L. (The man page of xmodmap contains some useful examples.)

To see your current keyboard mapping (which will show keycodes and corresp values assigned to keys) type

Code:
xmodmap -pke

To make these changes permanent you will then have to edit the keyboard map file.