Detect when an application goes fullscreen?
I am not sure if this solves your issue or not but the easiest way to detect whether or not a window is full screen would be to compare it's window dimensions to the window dimensions of your "desktop window". See below:

tell application "Finder" to set screenSize to bounds of window of desktop

tell application "Safari" to set windowBounds to bounds of front window

if windowBounds is equal to screenSize then
    return "It's Full Screen"
else
    return "Not Full Screen"
end if