If you've plugged an external monitor into your machine, Xorg's xrandr should tell you about it:
$ xrandr
Screen 0: minimum ..., current ..., maximum ...
eDP1 connected ... (normal left inverted right x axis y axis)
...
1024x768 60.0
800x600 60.0
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 connected ... (normal left inverted right x axis y axis)
...
1024x768 75.0
800x600 75.0
So we've got DPI and eDPI. We can also use xrandr to say output to the external monitor, DP1 in this case.
xrandr --output DP1 --mode 1024x768
This will clone your monitor. Add the argument --right-of ePD1 to make the new monitor extend the current.