Well, in my quick research this is unfortunately not something that Google has exposed to GAS. Now, what I am talking abou is the "checklist menu item" in Google Docs. This is where a box can be checked and you can format the text as "strike through" when checked. THIS is something that you can check for!!
My next question is, does this have to be a Document? Can you switch to a spreadsheet? The checkboxes in a spreadsheet show up as either true or false so you can definitely check for those.
AND as always... if anyone knows anything different PLEASE do not hesitate to let me know!! This is how we all learn... sharing our knowledge as we know it!!