Question : SavePage, detect if a save is needed

Hi, i'd like to provide visual feedback to the user that a "save page" is needed or not. What would be the best approach about it?

I have tried project.onChangeDocument but this only fires if you open a new document. I have also tried doc.history.onChangeCursor but when you take new actions cursor always stays the same (i guess because history goes like -1 -2 -3 and current cursor is always 0)

Best regards,