This is heavily dependent on the set up of the system. One way to get the information would be if xrandr is being used:
xrandr --query
This will display something like:
Screen 0: minimum 320 x 200, current 3046 x 1050, maximum 8192 x 8192
VGA1 connected 1680x1050+1366+0 (normal left inverted right x axis y axis) 473mm x 296mm
1680x1050 60.0*+
1280x1024 75.0 60.0
1152x864 75.0
1024x768 75.1 60.0
800x600 75.0 60.3
640x480 75.0 60.0
720x400 70.1
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 353mm x 198mm
1366x768 60.0*+
1360x768 59.8 60.0
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
DP1 disconnected (normal left inverted right x axis y axis)
You could then use some text processing tool to pull out the resolution for each display.