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