Hi Jrad37, for linux client the primary monitor is selected as the one located on (0,0) or the top/left -most one, if you want to use xrandr for checking, you can use cmd like the following ,
xrandr -q
here is some output from one of my machine
admin@LWT0080649eba45:~> xrandr -q -d :0.0
Screen 0: minimum 320 x 200, current 3200 x 1200, maximum 5120 x 2560
DFP1 connected 1600x1200+1600+0 (normal left inverted right x axis y axis) 641mm x 401mm
2560x1600 60.0 +
1920x1440 60.0
1920x1200 59.9
1920x1080 60.0 50.0 59.9 30.0 25.0 24.0 30.0 24.0
1600x1200 60.0*
1776x1000 50.0 59.9 25.0 24.0 30.0
1680x1050 60.0 50.0 59.9 24.0 24.0
1400x1050 60.0
1280x1024 75.0 60.0
1440x900 60.0
1280x960 75.0 60.0
1280x800 59.8
1152x864 60.0 75.0
1280x768 59.8
1280x720 60.0 50.0 59.9
1024x768 75.0 60.0
1152x648 50.0 59.9
1024x600 75.0 60.0
800x600 75.0 60.3
800x480 75.0 60.3
720x480 60.0 59.9
640x480 75.0 59.9 59.9
DFP2 connected 1600x1200+0+0 (normal left inverted right x axis y axis) 641mm x 401mm
1600x1200 60.0*+
1400x1050 60.0
1280x1024 75.0 60.0
1440x900 60.0
1280x960 75.0 60.0
1280x800 59.8
1152x864 75.0
1280x768 59.8
1280x720 59.8
1024x768 75.0 60.0
1024x600 75.0 60.0
800x600 75.0 60.3
800x480 75.0 60.3
640x480 75.0 59.9
CRT1 disconnected (normal left inverted right x axis y axis)
here, from "DFP2 connected 1600x1200+0+0"
we can parse "1600x1200" as resolution, "+0+0" as x ,y position
mac and windows client have relative position, http://doc.sikuli.org/screen.html is a good reference in my opinion maybe in-relative to this question.