Upside-down question mark
Solution 1:
Press CTRL+SHIFT while keeping them pressed, press and release U then type in 00BF, release CTRL+SHIFT, then you will have your ¿.
Source:W3
Solution 2:
set your keyboard layout to ES
setxkbmap es
to see places of characters on keyboard, run onboard
onboard
I hope it helps you.
to revert your keyboard layout run
setxkbmap us