I do essentially the same thing with my source documents which are usually pdf's. I use Adobe Acrobat and have made a checkmark stamp that I place next to the data on the pdf to indicate I have input that number on the appropriate form.

If you had a checkmark in ProSeries to indicate you have completed a particular form that could be useful, but it can also cause an over reliance on the checkmark. Sometimes data is added or deleted on other forms that rolls over to other forms and your checkmark would not be accurate in those cases.