Maybe you can check the keyboard events with xev. If the left shift is pressed you should see something like this:
KeyPress event, serial 47, synthetic NO, window 0x1e00001,
root 0xb7, subw 0xe0002e, time 1570704, (71,114), root:(1238,813),
state 0x0, keycode 50 (keysym 0xffe1, Shift_L), same ...