Open your terminal and type this:
$ xrandr -q | grep "connected"
If you have multiple monitors, you will have output as:
VGA1 connected primary 1280x1024+1920+0 (normal left inverted right x axis y axis) 338mm x 270mm HDMI1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 477mm x 268mm
Here, VGA1 and HDMI1 are your display. If you want to change the brightness of ‘VGA1’ type the following:
$ xrandr --output LVDS1 --brightness 0.9
The brightness ranges from 0.0 to 1.0 and 1.0 means full black.