"Send on Save" event Dialogue Box

Summary

Display dialogue box if "Save" button is clicked without checking "Send n Save"

Content

Hello,

We have checkbox "Send on save" in communication tab that ensures to send email to recipient when clicking "Save" button. I want to display dialogue box if "Save" button is clicked without checking "Send n Save" . So that user gets message on screen while saving that email won't be send. Because most of the users forget to check "Send on Save" and saves incident assuming that email is sent.
I couldn't find any rule specifying action on "Send on Save" checkbox. Kindly suggest some way to do so.