If I type:
$ xinput test 14
and press and hold return, I get:
key release 36
key press 36
key release 36
key press 36
...
until I release.
If you're using xev it shows KeyPress and KeyRelease events.
While hitting the Alt aka. Super key:
KeyPress event, serial 40, synthetic NO, window 0xae00001,
root 0x2ca, subw 0x0, time 595467354, (98,77), root:(102,443),
state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
XLookupString gives 0 bytes:
XmbLookupString gives 0 bytes:
XFilterEvent returns: False
KeyRelease event, serial 40, synthetic NO, window 0xae00001,
root 0x2ca, subw 0x0, time 595467453, (98,77), root:(102,443),
state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
XLookupString gives 0 bytes:
XFilterEvent returns: False