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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.