Connect additional monitor and check with xrandr on which display port its connected:

[mumrik@snusmumriken ~]$ xrandr -q
Screen 0: minimum 8 x 8, current 1366 x 768, maximum 32767 x 32767
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 277mm x 156mm
   1366x768      60.00*+
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.32    56.25  
   864x486       60.00  
   640x480       59.94  
   720x405       60.00  
   680x384       60.00  
   640x360       60.00  
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VGA1 connected (normal left inverted right x axis y axis)   
   1440x900       59.9 +   75.0     
   1280x1024      75.0     60.0      
   1280x960       60.0      
   1152x864       75.0      
   1024x768       75.1     70.1     60.0      
   832x624        74.6      
   800x600        72.2     75.0     60.3     56.2      
   640x480        72.8     75.0     66.7     60.0      
   720x400        70.1      
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

LVDS1 is the main display (laptop in this example). VGA1 is the newly connected additional monitor.

Switch additional monitor on, place right to the main display:

[mumrik@snusmumriken ~]$ xrandr --output VGA1 --mode 1440x900 --right-of LVDS1

Switch additional monitor off:

[mumrik@snusmumriken ~]$ xrandr --output VGA1 --off