Check mark counter

I was hoping that someone would be able to write a simple script for me. I am a teacher and grading pdf papers. I am using the check mark symbol under the "fill and sign". All I am looking for is a script that will just pop up a box that will tell me how many check marks I have placed on the document. Ideally I'd like to be able to amend the script to also count other characters from the fill and sign tool such as the cross or line. Is anybody able to assist me with this? Thank you very much.