X11 hacking

xev - displays information on events got by X windows (mouse move or keyboard press for example)

Example:

MotionNotify event, serial 31, synthetic NO, window 0x3a00001,
    root 0x59, subw 0x0, time 9142646, (124,15), root:(729,412),
    state 0x0, is_hint 0, same_screen YES

MotionNotify event, serial 31, synthetic NO, window 0x3a00001,
    root 0x59, subw 0x0, time 9142654, (126,9), root:(731,406),
    state 0x0, is_hint 0, same_screen YES

MotionNotify event, serial 31, synthetic NO, window 0x3a00001,
    root 0x59, subw 0x0, time 9142662, (128,3), root:(733,400),
    state 0x0, is_hint 0, same_screen YES

LeaveNotify event, serial 31, synthetic NO, window 0x3a00001,
    root 0x59, subw 0x0, time 9142670, (130,-3), root:(735,394),
    mode NotifyNormal, detail NotifyNonlinear, same_screen YES,
    focus YES, state 0
XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0.0"
      after 31 requests (31 known processed) with 0 events remaining.

look at the end of man xev for further reading.