I found the answer to this. It seems that there is no standardized value that is used to pass through to a checkbox. You will need to extract the acceptable value from the document. To do so I used the GeneralPdfUtils() and the getFieldType() method will return all the fields, their types, and accepted values.

So for my specific document the I9, a check box is marked by passing "1". Other documents may be true/false, YES/NO, On/Off.