I think you'll see this if you run in verbose mode.
e.g.
groovymame -v pacman > /tmp/pacman.txt
I have a WG D9200 but I set monitor type in mame.ini to arcade_15 and this is what I see in my log file:
[arcade@GroovyArcade ~]$ grep SwitchRes /tmp/pacman.txt
SwitchRes: v0.015f, Monitor: arcade_15, Orientation: horizontal, Modeline generation: enabled
SwitchRes: Monitor range 15625.00-16200.00,49.50-65.00,2.000,4.700,8.000,0.064,0.192,1.024,0,0,192,288,448,576
SwitchRes: Found output connector 'VGA-0'
SwitchRes: Entering switchres_modeline_setup
SwitchRes: v0.015f:[pacman] Calculating best video mode for 224x288@60.606060 orientation: rotated
SwitchRes: ( 1)x( 1)_(60=0.0000Hz)
SwitchRes: [pacman] (1) vertical (224x288@60.61)->(656x488@60.61)
SwitchRes: Modeline "656x488_60 16.09KHz 60.61Hz" 13.90 656 688 752 864 488 491 497 531 interlace -hsync -vsync
SwitchRes: Running 'xrandr --newmode "656x488_60.61" 13.90 656 688 752 864 488 491 497 531 interlace -hsync -vsync'
SwitchRes: Running 'xrandr --addmode VGA-0 "656x488_60.61"'
SwitchRes: Running 'xrandr --output VGA-0 --mode "656x488_60.61"'
Hope this is helpful.